Why is RGB called additive color

Light, colors and color models (RGB, HSB, CMYK)

In photography and in the printing sector, one has to deal with color models such as RGB, HSB or CMYK on a daily basis. On this page we deal in detail with the properties of such color models, how to visualize them, how to use them and what advantages they each have. Before we dive into the color models, however, we take care of the creation and structure of light.

The spectral colors of light

What is light actually? Mankind has been asking this question since time immemorial. Does a beam of light consist of tiny particles or is light simply a wave, as can be received by a simple radio or two-way radio?

There is a natural phenomenon that has always fascinated mankind and which still inspires enthusiasm among young and old today: a magnificent, wide-span rainbow on the sky horizon. This colorful play of colors, where you can see colors from purple to green and yellow to red flowing into each other, is always nice to see. However, it was not until the 17th century that this phenomenon could be scientifically interpreted.

Isaac Newton was the great tinkerer who could split white sunlight into its spectral colors on a glass prism. From then on it was known that white light is a mixture of a multitude of individual colors; Blue light is very strongly refracted on a prism; red light, on the other hand, is only slightly refracted. In between there are an infinite number of gradations. This creates the light spectrum from violet to blue, green, yellow and orange to red.

The effect that Newton achieved with a glass prism takes place in the same way with a rainbow. Small water droplets or ice crystals refract the light just like a glass prism.

Light as an electromagnetic wave

Through Isaac Newton's prism experiments, people became aware that white light is made up of many individual colors or that white light can be broken down into its spectral colors. But what actually is light, where does light come from, how does light spread?

In the 19th century, the physicists James Maxwell and Heinrich Hertz succeeded in showing that light is an electromagnetic wave. But what exactly is an electromagnetic wave? According to James Maxwell, an electric field can generate a magnetic field and vice versa. In the case of an electromagnetic wave, electrical field energy and magnetic field energy convert cyclically into one another; In contrast to sound or water waves, electromagnetic waves do not need a carrier, i.e. an electromagnetic wave can propagate in a vacuum.

So light is an electromagnetic wave that can also propagate in a vacuum. The speed of propagation of such an electromagnetic wave is approx. 300,000 km / s, i.e. the value that is referred to as the speed of light. The speed of light is so high that it cannot be seen with the eye. A ray of light would only take a little more than a tenth of a second to orbit the earth. Even as far as the moon (approx. 400,000 km from the earth), a ray of light travels just over a second. The speed of light only becomes "slow" when one thinks astronomically. The light that our sun sends to us every day is at least 10 minutes on the move, as the mean distance between the sun and the earth is around 150 million kilometers. The closest star to our solar system Alpha Centauri is over 4 light years away from us. The indication in kilometers would produce a barely legible, large number that the distance is only given in light years. And even this star is still around the corner, if you look at the dimensions of the universe.

Light as part of electromagnetic radiation

Light consists of electromagnetic waves that travel at the speed of light. Now someone may rightly point out that an electromagnetic wave is more like a wave that a simple radio or radio device receives. Electromagnetic waves are also the waves that warm food in a microwave oven; While such waves are invisible, light is something visible!

In fact, a light wave does not differ from a radio wave or a microwave in its nature but only in terms of its wavelength or frequency. Everyone knows that you can receive FM radio stations in the frequency range between 87.5 and 108.0 MegaHertz. Radios have even lower frequencies. That is why electromagnetic waves in this frequency range are also referred to as radio waves. Everyone knows waves with frequencies between 20 Hz and 30,000 Hz: sound waves.

In the gigahertz range of electromagnetic radiation one can find the well-known microwaves, which, however, are by no means restricted to kitchen appliances; The satellites of the GPS navigation system also transmit in the microwave range.

If you keep increasing the frequency of electromagnetic radiation, you finally get into the range of X-rays, then into that of gamma radiation (radioactive radiation) and from frequencies above 1022 in the realm of cosmic rays.

That part of the electromagnetic radiation that has a wavelength between 100 nanometers and 1 millimeter or a frequency between 1011 and 1016 Hertz is called light. Now it becomes clear why light is only a small part of the spectrum of electromagnetic radiation.

Even light can again be divided into a multitude of individual areas. The range of visible light is only a small part of what is commonly referred to as light. Visible light has a wavelength between 380 and 780 nanometers. An electromagnetic wave with a wavelength of 380 nanometers can therefore be perceived as visible violet light with the eye. An electromagnetic wave with a wavelength of 780 nanometers can be perceived as red light. In between is the entire visible color spectrum from violet to blue, green, yellow, orange to red.

But what is beyond the limits of visible light in the area that is also called light? If you reduce the wavelength further, you get the well-known ultraviolet light (UV-A, UV-B, UV-C), which is known to ensure a healthy holiday tan. In the other direction comes the infrared range, also known as thermal radiation.

Finally, let's clarify the question of how the wavelength and the frequency of electromagnetic waves are related. It is the speed of light, which is the quotient of the distance covered (wavelength λ) and time (reciprocal of frequency f). Using the speed of light, we can calculate the frequency associated with a certain wavelength. Example radio frequency with 108 MHz: λ = c / f = 300,000,000 / 108,000,000 = 2.8m. Example red light with a wavelength of 700 nm: f = c / λ = 300e6 / 700e-9 = 4.3e14 1 / s.

How many colors are there?

Visible light is that part of the electromagnetic radiation that has a wavelength between 380 and 780 nanometers. Light with a wavelength of 380 nanometers has a violet color, light with a wavelength of 780 nanometers has a red color. In between is the visible light spectrum. The two limits 380 and 780 may initially lead to the assumption that there are 400 individual spectral colors. In fact, an electromagnetic wave can also have the wavelength of 475.162 nanometers. So you can see that in the range of visible light there are theoretically an infinite number of spectral colors.

White sunlight contains all spectral colors in the visible range. One therefore speaks of a continuous color spectrum. A single spectral color with a very specific wavelength can only be generated with a laser.

So there are theoretically an infinite number of individual spectral colors with different wavelengths. In nature, however, you never see a single spectral color, but always mixtures. From any mixture of any number of spectral colors, any number of new colors can arise, so there are in fact an infinite number of colors! However, this does not mean that the human eye can perceive or even differentiate between all possible color tones.

Color and color perception

If I wrote in the previous chapter that light with a wavelength of 780 nanometers has a red color, this is not quite correctly expressed. After all, color is not a physical quantity, but a physiological phenomenon. If an electromagnetic wave with a wavelength within the range of visible light falls into our eye, a Color stimulus triggered our brain to a certain extent Color sensation processed. While an electromagnetic wave with a certain frequency can be clearly described physically, color is not an absolute physical quantity, but a perceived sensory stimulus. Example: There are optical illusions in which our brain perceives more colors than are actually there.

What does it mean if, for example, a flower pot is red? From a physical point of view, the flower pot has no color at all but only a certain absorption and reflectivity when it is illuminated by a light source. After all, in a dark room, the flower pot is just as black as the surroundings. The moon is also black in the sky when it is not illuminated by the sun.

If a flower pot that appears red is illuminated by white sunlight, this means that the surface of the flower pot reflects the red components of the light mixture, while it absorbs the remaining light components. This is how the red color sensation arises.

What are color models used for?

An infinite number of lights can be created from an infinite number of spectral lights by random mixing. I am now consciously speaking of lights, i.e. electromagnetic waves, and not of color sensations. In no computer or in no optical-digital instrument can an infinite number of lights be represented or differentiated. For this reason, color models are used that cover the largest possible range of possible color sensations.

From the CIE (Commission Internationale de l'Eclairage) there is the famous CIE diagram, on the boundary curve of which there are all spectral lights and inside which all possible mixtures of these spectral lights are located. This diagram contains all possible lights and light mixtures that can arise from the spectral lights.

A color model could now look like, for example, that one selects 100 or 1000 points on the edge curve of this CIE diagram and represents a color as a finely graduated mixture of these 100 or 1000 points. Such a color model would cover a large part of the CIE color model. In practice, a point on the screen would have to be composed of 100 or 1000 individual lamps, graphics cards would have to be the size of a mainframe, and 100 color cartridges would have to be used in the inkjet printer.

The color models described below do not cover the entire possible color space, but are much easier to describe and put into practice. It should be said in advance that there are many color models, all of which have their justification and their advantages and disadvantages. They are all just models that want to describe reality as simply as possible but still as precisely as possible.

The additive RGB color model

With the RGB color model (RGB = R.ot, Grun, B.lau), all colors of the RGB color space are combined additively from the three basic colors red, green and blue (RGB). So you only use 3 basic colors to create all other colors by mixing them. If you mix red and green in equal proportions, you get yellow; red and blue make magenta; blue and green make cyan. If you mix all three basic colors, you get white; if you switch off all three basic colors, black remains. The three basic colors red, green and blue are also called primary colors; the colors that are created by mixing the primary colors are also known as mixed colors.

Since mixed colors always result from the superposition of several primary colors, mixed colors are always lighter than the basic colors. Example: Yellow is created by superimposing red and green; yellow is therefore lighter than red or green; it is ultimately created by the intensity of two light sources. Whenever the three primary colors are superimposed with the same intensity, e.g. 30% red, 30% green and 30% blue, a gray color is created. On a gray scale from 0% to 100%, 0% corresponds to pure black, i.e. the RGB values ​​are each zero, 100% corresponds to light white, i.e. the RGB values ​​are each a maximum. In between there are gray values, which are also referred to as achromatic colors.

The RGB color model is based on the human eye, where different receptors on the retina react to these three basic colors. Active radiating devices such as a television set, a scanner or a digital camera work according to the RGB color model. In a television set, a pixel is created by three lights with the colors red, green and blue. If all the lights are off, the picture is black; if all the lamps light up at full strength, a white image is obtained. With a scanner, the original forms the active light source, with a digital camera it is the radiant motif.

The two adjacent pictures show this with a practical example. These are photos that were taken with a digital microscope. A high-resolution computer LCD screen was photographed. In the left picture we can see how blue, red and green pixels line up and glow strongly. A white area is shown on the screen here. In the right picture we see that only the red subpixels are lit, the green and blue are off, i.e. black; a red area is shown on the screen. A computer screen, which has a resolution of 1900 x 1280 pixels, represents each individual pixel with the help of three such sub-pixels, which glow differently depending on the color to be reproduced. Of the more than 2 million pixels that such a monitor displays, only approx. 60 points are shown in the adjacent images.

The RGB color model is a device-dependent color model. One can describe a dark red numerically with an RGB value of e.g. (150,0,0); how this red actually appears depends on the device in question. 10 televisions display the red value (150,0,0) differently, also depends on the color settings on the respective device. 10 scanners also record a certain red value individually, i.e. depending on the device.

With the three basic colors red, green and blue, a large number of different shades can be represented by different mixing ratios; On a computer monitor, for example, a color depth of 24 bits is used, i.e. 8 bits per color channel. This means that both the red, the green and the blue light of a pixel with an 8-bit color depth can display a total of 256 color tones. In total you get 256³, ie approx. 16 million colors. In practice, an RGB value is then represented as a number triple with a value range from (0,0,0) to (255,255,255). Devices with a color depth of 16 bits per color channel (e.g. high-quality film scanners) then have a value range between (0 , 0,0) and (65536,65536,65536).

I would like to emphasize once again that the colors in the RGB color model are made up of the basic colors. To make this fact clear to yourself, you put yourself mentally in space, where there is nothing and darkness prevails. If a lamp emits red light, then nothing turns into red. If a second lamp emits green light on the red one, then red and green add up to yellow. Finally, if a third lamp shines blue light on the yellow, a white color is obtained. So black is the color that characterizes nothing, white is the color that characterizes everything.

How can you easily visualize the additive RGB color model? Since all possible colors of the color model are made up of the three basic colors red, green and blue as mixed colors, a three-dimensional representation in the form of a cube is obvious. At the origin of the three-dimensional coordinate system is the color black with an RGB value of (0,0,0).

The three color axes red, green and blue start from this origin. At the end of the three axes (marked by small circles in the graphic) are the three pure primary colors red = (255,0,0), green = (0,255,0) and blue = (0,0,255). The other corners of the cube correspond to the mixed colors yellow = (255,255,0), cyan = (0,255,255), magenta = (0,255,255) and white = (255,255,255). All colors of the RGB color model are either on the edges, the outer surfaces or inside the cube. The diagonal from the origin black = (0,0,0) to white = (255,255,255) corresponds to the gray scale with the achromatic colors.

The representation of the RGB color model in the form of a cube enables the three-dimensional visualization of all RGB colors, but in practice a two-dimensional representation in the form of a color hexagon is preferred. The color hexagon is obtained from the color cube by placing the color cube on the black corner and pressing the cube almost flat. The graphic opposite shows how a color hexagon with the corner colors red, yellow, green, cyan, blue and magenta is created.

In the middle of the hexagon is black or a shade of gray. The color hexagon can also be imagined as a cut through the color cube. Depending on which plane you cut at, you have a gray value in the middle in the range from black to white.

It is only a small step from the color hexagon to the widely used color wheel: You simply round off the edges of the color hexagon and you get a color wheel with the 6 basic colors red, yellow, green, cyan, blue, magenta, each at an angular distance of 60 ° to each other. In the middle of the color wheel is an achromatic color, i.e. a shade of gray between black and white. The angle in the color wheel characterizes the hue, the distance from the center of the circle characterizes the saturation. However, this brings us to the HSB color model below, which is derived directly from the RGB color model.

The HSB color model

The RGB color model described above is ideal for describing a color in terms of numbers. With an RGB value of (200,200.0) it is easy to imagine the components of the hue, and the RGB color (200,200.0) can be reproduced in different computer programs on different output devices. However, when it comes to RGB color tones, it is often difficult to describe a color from the numerical value. Who can describe what color the value (217,150,97) stands for? And it is even more difficult to compare colors in the RGB color model: How do the colors (217,150,97) and (199,168,102) differ?

The HSB color model is used to describe colors more easily and to compare them with one another. The letters mean HSB Hue (hue), S.aturation (saturation) and B.rightness (brightness). The hue is described as an angle between 0 ° and 360 °, while the saturation and lightness are given as percentages between 0% and 100%. Even if the description of a color in the form of hue, saturation and brightness seems a bit strange, these descriptions are used in everyday life, even if one is not aware of them. Who, for example, the color of his car as bright neon blue describes, gives with the word blue the hue, with neon the saturation and with bright the brightness on. In numerical values, this color could look like this: HSB = (240 °, 80%, 90%).

In the HSB color model, the hue (hue) is specified as an angle on a color wheel. The 6 primary colors red, yellow, green, cyan, blue and magenta are applied counterclockwise at a distance of 60 °. Examples: An angle of 60 ° corresponds to the color yellow, at an angle of 120 ° the color appears green, and at 240 ° the color appears blue. Two color tones can be compared with one another using the angle difference. yellow and green have an angle difference of 60 °, so they are still very similar. Yellow and blue have a difference angle of 180 °, the greatest possible difference between two color angles. Yellow and blue therefore differ as much as possible, which is why they are also referred to as complementary colors. Other complementary colors are, for example, red and cyan or green and magenta.

If two color tones have an angle difference of 180 ° in the color wheel, then the two color tones are complementary to each other; one therefore speaks of complementary colors.

The HSB color model can be imagined either as a cylinder or as a sphere, whereby the color wheel just described corresponds to a cross section of the barrel or sphere. The hue is plotted against the angle on the color wheel; the radius of the color circle corresponds to the saturation, where the radius 0 corresponds to a saturation of 0% and the maximum radius corresponds to a saturation of 100%. Colors with a saturation of 0% are called achromatic colors; they are shades of gray from black to white. So there is a shade of gray in the middle of the color wheel. If the cylinder or the ball is cut open at the bottom, it is dark gray (at the bottom black); a cut in the upper area is a light gray (white at the top). In everyday language, saturation is saturation with terms such as pale, dull, dull, neon or bright described.

Colors with a saturation of 0% are called achromatic colors. These are gray tones from black (brightness = 0%) to white (brightness = 100%).

The brightness is also specified in the HSB color model as a percentage value between 0% and 100%. In the cylinder model, the brightness goes from 0% (black) to 100% (white) from bottom to top, whereby the cylinder axis corresponds to the achromatic colors, i.e. forms a gray scale between black and white. The cylinder covers are completely black or completely white; the achromatic colors black and white have neither a hue nor a saturation. The sphere model visualizes this fact better, since the poles of a sphere have neither a radius nor an angle.

The HSB color model, like the RGB color model or the CMYK color model, is device-dependent. With the help of an ICC profile, however, device properties can be described and compensated. While the RGB color model reproduces colors based on the mixing ratio of red, green and blue in numerical values, the HSB color model describes colors based on color properties (hue, saturation and lightness) that can easily be compared with one another.

The HSB color model is particularly suitable for the intuitive description of colors and for comparing colors with one another.

Comparison of RGB color model and HSB color model

In the previous chapters we got to know the RGB color model and the HSB color model. The same colors can be described with both color models, whereby in the RGB model one color is described as a triplet of numbers from the three basic colors red, green and blue, each in the value range between 0 and 255, while in the HSB model a color is described as a triplet of numbers from hue (0 ° -360 °), saturation (0% -100%) and brightness (0% -100%).

Both color models are device-dependent. This means that colors are clearly described numerically, but that it does not determine how the color is actually displayed on an output device (e.g. screen). In order to describe colors absolutely with the RGB color model or HSB color model, an ICC profile is necessary that relates the device-dependent colors to a device-independent color model. Details on this topic can be found on our Introduction to Color Management page.

The RGB color model describes colors as a mixture of the three basic colors R.ot, Grün and B.lukewarm. RGb colors are therefore particularly suitable for devices such as screens or displays that display almost any color exactly on the basis of these three basic colors. The HSB color model describes colors more intuitively from three components Hue (hue), S.aturation (saturation) and B.rightness (brightness). While a technical device such as a screen can immediately display a color with an RGB value, humans can do less with an RGB value such as (217,150,97). Conversely, a computer monitor cannot display an HSB value of e.g. (240 °, 80%, 90%), while a person can easily imagine a certain shade of blue.

The RGB color model is suitable for technical devices such as computer displays, where color tones are created as a mixture of the 3 basic colors red, green and blue. The HSB color model is more suitable for intuitive color description and for color comparisons.

Finally, let's do a practical test with Adobe® Photoshop® in order to be able to compare color tones in the RGB model directly with color tones in the HSB model. The professional version of Photoshop® (CS versions) offer color pickers that display both the RGB values ​​and the HSB values ​​for individual colors. In the Elements version only the RGB values ​​are displayed.

hue

Let's start with the hue in order to understand the relationship between the basic colors and the color wheel angles. We first sample the primary colors on the color wheel by specify different hue angles for colors with 100% saturation and 100% lightness. The 6 figures opposite show how the primary colors red, yellow, green, cyan, blue and magenta are represented for the color wheel angles 0 °, 60 °, 120 °, 180 °, 240 ° and 300 °. These basic colors have the well-known RGB values ​​red = (255,0,0), yellow = (255,255,0), green = (0,255,0), cyan = (0,255,255), blue = (0,0,255) and magenta = (255,0,255). The hue is varied in the left square color field over the horizontal x-axis. Let's hold on:

A pure hue in the HSB model (brightness = 100%, saturation = 100%) is more than a maximum in the RGB model two Values ​​specified, e.g. (255,50,0) or (255,0,0).

If you slide the slider in the square color field slowly from left to right, you can see that only the H value changes with the HSB values, while only two numbers change with the RGB values; the third remains constant at 0.

brightness

Now let's vary the brightness at 100% saturation and a color angle of 0 °, i.e. a red hue. The adjacent figures show the three brightness values ​​100%, 75% and 50% for the red tone. While at a brightness value of 100% the red tone is brightly shining, a 75% brightness results in a dark red, while at 50% brightness one speaks only indirectly of red, e.g. of reddish brown. Based on the associated RGB values, it is easy to see how the variation in brightness affects: 100% brightness corresponds to a maximum red of (255,0,0), whereby the red value with decreasing brightness over (192,0,0) to ( 128,0,0) decreases. If you reduce the brightness further down to 0%, the red-brown changes to black (0,0,0).

If we vary the brightness at 100% saturation for the color yellow (color angle 60 °), then we recognize a darker yellow-green RGB = (230,230.0) and at 60 from a light yellow RGB = (255,255.0) at 90% brightness % Brightness results in a maple green color RGB = (178,178,0). Here, too, if the brightness is reduced further, the RGB values ​​change to black (0,0,0). The brightness is in Photoshop® set via the vertical color bar from 0% (below) to 100% (above).

In the RGB model, the brightness of a color tone is determined by varying the maximum two Numerical values ​​are set that characterize the pure color tone.

If you move the vertical brightness regulator up and down, only the number for the brightness B changes with the HSB values, while with the RGB values ​​only the two numerical values ​​that characterize the hue change; the third number in the RGB values ​​remains unaffected.

saturation

With the variation of the hue in the HSB color model, we simply recognized the effects in the RGB model: The corresponding red, green or blue tones were assigned values, e.g. (255,0,0) for red or (0, 0.255) for blue. When the brightness was varied in the HSB model, the corresponding numerical value for the hue in the RGB model was changed, e.g. from (255,0,0) at half brightness (128,0,0). But how does the variation in saturation in the HSB color model affect the values ​​in the RGB model?

Let us look at the adjacent figures, in which the saturation for the hue red (H = 0) at maximum brightness (B = 100) is gradually reduced from S = 100 to S = 90 to S = 70. We can see that the bright red turns more and more into a duller flamingo red at maximum saturation. When looking at the associated RGB values, we see that the pure red (255,0,0) at 100% saturation becomes a mixed color of (255.25.25) at 90% saturation and (255.76, 76) at 70% saturation. The R.The ot value therefore retains its maximum value of 255 as a result of the 100% brightness; with decreasing saturation, however, green and blue color components are added in the same ratio (i.e. cyan_components). Cyan corresponds to the complementary color of red. So let's keep it simple:

When the saturation is reduced from 100% to any value, proportions of the complementary color corresponding to the pure color tone are added.

Let's do the same experiment with the hue yellow (H = 60) at maximum brightness (B = 100). The adjacent figures show how this is powerfully at a saturation of S = 100% bright yellow becomes paler with decreasing saturation. The associated RGB values ​​show how more and more shares of the complementary color blue are added with decreasing saturation. From the images we can also see how the saturation in Photoshop® is visualized: It can be set in the square color field via the vertical axis, with a saturation of 0% at the bottom and 100% at the top.

The subtractive CMYK color model

The RGB color model is a simple model to describe the mixture of light from different light sources. In nature, however, color impressions arise not only from light sources but also - and even mostly - from reflection and absorption. A yellow flower on a green meadow does not appear yellow to us because it actively emits yellow light, but because it absorbs and reflects the white sunlight falling on it in such a way that it creates a yellow color impression; No light falls on the flower at night, so we don't see it either; it doesn't shine by itself. What does this mean in detail? The flower absorbs the sunlight, which is made up of the entire color spectrum, most of the colors except for the yellow; it is reflected and perceived by the viewer.

The color yellow is not created by additive color mixing of the basic colors red and green in the RGB color model, but by subtractive color mixing. Simply put, the red and green components are subtracted from the white sunlight, leaving yellow. The CMYK color model was created on the basis of such a subtractive color mixture. In order to understand the additive RGB color model, we put ourselves in space, where we actively brought light into nothingness with light sources. To understand the CMYK color model, let's stay on earth, where white sunlight can be found during the day; So first of all everything (all shades) is there.

If you completely filter out the red components from the white sunlight, the color cyan remains. If you filter out all the green parts from the white sunlight, the color magenta remains. The filtering of all blue components results in a yellow color. The CMYK (cyan, magenta, yellow) color model is made up of these three colors. The complete color palette is obtained by mixing these three basic colors in different nuances.

What kind of color do you get when you mix two CMYK basic colors like magenta and yellow? Magenta is created by subtracting the green part of the sunlight. Yellow is created by subtracting the blue components from the white sunlight. If the green and blue components are filtered out of the sunlight, only the color red remains. When adding the two CMYK colors magenta and yellow, the subtractive effects of both colors add up and the result is the color red. The addition of cyan, magenta and yellow means a subtraction of red, green and blue from the white sunlight; there is nothing left, so black.

Now it should also become clear why one does not find red, green and blue color cartridges in an inkjet printer but the colors cyan, magenta and yellow. As with sunlight, the starting point is white paper, which reflects light components; If you apply the colors cyan, magenta and yellow on top of each other, the result is a black hue. However, since this black is not black enough, pure black is used as an additional fourth color or ink cartridge. Hence the name CMYK (K stands for key color). All colors of the CMYK color model lie within the cube or on the cube edges or cube corners.

The CMYK color model, like the additive RGB color model, can be visualized as a color cube. The bottom corner of the cube contains the basic color white (0,0,0,0); The three axes cyan, magenta and yellow run from this origin. At the end, these each have the value cyan = (255,0,0,0), magenta = (0,255,0,0), yellow = (0,0,255,0). The diagonal of the cube contains the achromatic colors, i.e. gray tones between white and black. Black is at the end of the diagonal and has the maximum value of the three basic colors black = (255,255,255,0) or the maximum value of the key color (0,0,0,255).

The subtractive CMYK color model, like the additive RGB color model, is device-dependent. With a CMYK value of for example (0,0,255,0) one can clearly describe a yellow color tone, however how it appears on a printer depends on the printer, the inks used and the printer paper.

Paints and coloring agents

If I illuminate a wall with a red, green and blue studio spotlight, an almost pure white spotlight is created, as it should be according to the RGB color model. But what kind of color results when I paint a white sheet of paper first with a red, then with a green and finally with a blue felt-tip pen, perhaps a white sheet again? You never get any light shade, but an unsightly brown-black color.

At this point I would like to point out the improper use of the word colour in the German language. In our language, the word color is used to designate both Color sensation as well as one Colorant used. The color sensation is what our brain makes of it when electromagnetic radiation in the range of visible light hits the eye. A colorant is what we use to paint a wall or to paint a sheet of paper. The English language makes a clear distinction between color perception and colorants: You buy in a hardware store paint to paint a room. The walls then have a color color.

Back to the above question: A headlight is an active light source that emits a very specific light that is superimposed with the other headlight lights to form a new color shade according to the RGB color model. A red felt-tip pen is a colorant which, when applied to a sheet of paper irradiated with sunlight, absorbs the blue and green components of the light according to a subtractive color model and thus only reflects the red components. The same applies to the green and blue felt-tip pens. If you apply these three colorants to each other, virtually all the light components of the sunlight are absorbed and then, according to the subtractive color model, an almost black color is created.

While lights mix additively, colorants are mixed subtractively, i.e. when two colorants are mixed, their subtractive effect is added.

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