Trichromat

A trichromat is an organism for which the perceptual effect of any arbitrarily chosen light from its visible spectrum can be matched by a mixture of no more than three different pure spectral lights. The condition of being a trichromat is called trichromacy.

The normal explanation of trichromacy is that the organism's retina contains three 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 three, since different types may be active at different light intensities. In vertebrates with three types of cone cells, at low light intensities the rod cells may contribute to colour vision, giving a small region of tetrachromacy in the colour space.

Humans and other Old World primates are usually trichromats, as are female New World monkeys of most species, and both male and female howler monkeys. Recent research suggests that trichromacy may also be quite general among marsupials. Most other mammals are currently thought to be dichromats, with only two types of cone (though the possibility of limited trichromacy at low light levels where the rods and cones are both active). Some species of insects (such as honeybees) are also trichromats, being sensitive to ultraviolet, blue and green instead of blue, green and red.