The brain receives signals from three different color channels: red, green and blue. However when the brain receieves a mix of signals, such as a little bit of red and a little bit of green, we perceive colors that aren't red, green or blue, but mixtures of these three primary colors. When red and green signals are produced, they combine through color addition to form the signal interpreted as yellow.    Pictured at right are sensitivities of the three secondary colors: cyan, magenta, and yellow. These colors are formed by adding equal amounts of two primary colors together.
 Because the only color signals the brain receives are red, green and blue stimuli, all colored visible light can be expressed as the mixture of a certain amount of red, green, and blue light. This picture shows what colors are created when equal amounts of red, green, and blue are mixed.

Television screens and computer monitors use color addition to create the many colors you see on the screens. If you look very very closely at a monitor or TV, you will see many sets of tiny red, blue, and green lights. When these lights combine and strike your eye, you interpret the red, blue and green combination to be color.

The "Color" White

White is the combination of all colors of light. In other words, the color white is seen when the red, green and blue channels all send signals to the brain. It follows that red light plus green light plus blue light produces white light. However red plus green without the blue produces yellow. By deduction, yellow light plus blue light most produce white light.

Colors that combine together to form white light are called complimentary colors. . There are many sets of complimentary colors such as blue and yellow, green and magenta, and red and cyan. Notice that these colors are completely opposite: there is no such thing as a blueish yellow, greenish magenta, or reddish cyan.

 There are two easy ways to find a color's compliment. On the chromaticity diagram, find the color equidistant from and opposite from the white in the center of the diagram. The other method is to use the RGB Table above: reverse the amounts of red, green and blue in the color. For example, the compliment of 42% red, 58% green, 70% blue is 58% red, 42% green, 30% blue. In both cases the idea is the same: to find two colors that together will completely and equally stimulate all three channels in the eye.

Complimentary colors are the colors seen in negative afterimages. If you stare at the color green for a long time, your "green" channel is desensitized. When you look at something white, the green channel doesn't respond as strong as it would under normal circumstances, so the white appears like green's compliment: magenta.

Yellow

The ideas of color addition create a very interesting question about the way we view the world. We perceive red and green together as being yellow, however yellow by itself is also a spectral color. Polychromatic red plus green may appear the same as monochromatic yellow, but the mixture is not actually the same color.

Light combinations that appear the same but are actually different are called metamers. Metamers exist because we only have three color channels to determine color. If we had more channels there could be more color signal combinations and we could see more colors.

A Word on Subtraction:

Many people reading this may be shocked that we called the primary colors red, green and blue. Everyone learns in preschool that the primary colors are red, yellow and blue. This set of primary colors is also true (almost). These are the primary colors of pigments, not of light because of the process of color subtraction. Pigments are the dyes, paints, oils, and inks that can be used to color items. When light hits a pigment, some of it is absorbed, or subtracted. When white light strikes a black surface it is all absorbed so no light reflects and hits the eye. When white light strikes a green surface the red and blue are subtracted, leaving green light to strike the eye.

When different pigments are mixed, they absorb different wavelengths of light. Blue pigment absorbs red and green, while red pigment absorbs blue and green. When the two are mixed, they absorb most of the green, and some blue and red. The blue and red that reflect add to form a magenta or purple color.

So why are the primary colors of pigment red, yellow and blue? In fact, they're not... quite. The primary colors are magenta, yellow, and cyan, the familiar complements to the primary colors of light. Why? Because when white light strikes any of these pigments, only their complements will remain. Just as all color can be measured in terms of how much red, green, and blue are added to black (no color), it can also be measured in how much cyan, magenta, and yellow are taken away from white (all colors).

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One last word on the interesting color yellow. It is easy to visualize two of the mixes of the primary colors of light. Cyan can easily be seen as a greenish-blue. Magentas and purples are blueish-reds. But our perception of yellow is completely different from our perception of either blue or green. Using the color picking experiment above, you can see that yellow plus a little blue is simply a lighter yellow and blue plus a little yellow is simply a lighter blue. The reason why we see yellow and blue as completely different colors is still unknown today.

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