Dichromat

A dichromat is an organism that can match any color they see with a mixture of no more than two pure spectral lights. By comparison, a trichromat requires three pure spectral lights to match all colors in their visual spectrum. The condition of being a dichromat is called dichromacy.

Physical causes of dichromacy
The normal explanation of dichromacy is that the organism's retina contains two types of color receptors (called cone cells in vertebrates) with different absorption spectra. In practice the number of such receptor types may be greater than two, since different types may be active at different light intensities (in other words, the absorption spectra of different types of color receptors may overlap). In vertebrates with two types of cone cells, at low light intensities the rod cells may contribute to color vision, giving a small region of trichromacy in the color space.

Dichromacy in mammals
It is currently believed that most mammals are dichromats. The straightforward exceptions are Old World primates, including humans, which are usually trichromats, and sea mammals (both pinnipeds and cetaceans) which are monochromats. New World monkeys are a partial exception: in most species, males are dichromats, and about 60% of females are trichromats, but the owl monkeys are monochromats, and both sexes of howler monkeys are trichromats. Recent research (e.g. Arrese et al, 2005) suggests that trichromacy may be widespread among marsupials.

Human dichromats and color blindness
The two best-known forms of color blindness in human is the result of dichromacy, since one of the three cone systems is non-functional in these conditions. However, many people who are described as color blind are in fact anomalous trichromats; in this condition, there are three functional cone systems but one of them has an unusual absorption spectrum so the person does not make the same color matches as the rest of the population.