The three types of color-sensitive cone cells on the human retina each have definite response curves as a function of wavelength. The perceived color of light with a given wavelength distribution is basically determined by the three numbers obtained by integrating these responses. For any wavelength distribution it turns out that if one scales these numbers to add up to one, then the chromaticity values obtained must lie within a certain region. Mixing n specific colors in different proportions allows one to reach any point in an n-cornered polytope. For n = 3 this polytope comes close to filling the region of all possible colors, but for no n can it completely fill it—which is why practical displays and printing processes can produce only limited ranges of colors.
An important observation, related to the fact that limitations in color ranges are usually not too troublesome, is that the perceived colors of objects stay more or less constant even when viewed in very different lighting, corresponding to very different wavelength distributions. In recent years it has become clear that the origin of this phenomenon is that beyond the original cone cells, most color-sensitive cells in our visual system respond not to absolute color levels, but instead to differences in color levels at slightly different positions. (Responses to nearby relative values rather than absolute values seem to be common in many forms of human perception.)
The fact that white light is a mixture of colors was noticed by Isaac Newton in 1704, and it became clear in the course of the 1700s that three primaries could reproduce most colors. Thomas Young suggested in 1802 that there might be three types of color receptors in the eye, but it was not until 1959 that these were actually identified—though on the basis of perceptual experiments, parametrizations of color space were already well established by the 1930s. While humans and primates normally have three types of cone cells, it has been found that other mammals normally have two, while birds, reptiles and fishes typically have between 3 and 5.