If you go to any course about photography, you learn that all colors are made up of the colors red, green and blue (RGB). (If you mix red, green and blue light, you will get white light, or any other color according to how much of each light color you use.)
If you ask a physicist, he or she will tell you that the visible light is made up of all the colors in the rainbow. The water splits the white light and shows the rainbow with all the different colors it contains.
What is color, and what is it made of? Are all the colors in the rainbow really basic colors, and photographers have just experienced that you can reproduce all those colors with red, green and blue? What is the commonly accepted idea about colors?
Answer
This has as much to do with biology as with physics. The long answer on the biology is here. The biology in summary: the human eye has only three different "color-sensitive" elements, and uses a complex combination of the amount of response it sees from each of these to assign a "color" to the image.
Because there are only three sensitivities in the human eye, there are a variety of different techniques using only three basic (in some schemes called primary) colors to represent colors to humans.
The actual frequency of the light emitted from the part of the rainbow we call green may have the same effect on the human eye as something we get by mixing our blue crayon with our yellow crayon on a piece of paper. But it is easy to build a detector which will trivially differentiate between monochromatic green from one slice of a rainbow, and a mixture of the light reflected from blue and yellow crayon pigments.
In principle, humans might have evolved a different eye with four or five different "color" detectors, in which case the schemes needed to make color images would probably need to have four or five basic colors, and the images we see from our current three color representations would seem to be washed out, missing something important. But the eye didn't develop that way.
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