Dichromacy

Dichromacy in humans is a fairly severe color vision defect in which one of the three basic color mechanisms is absent or not functioning. It is hereditary and sex-linked, predominantly affecting males. Dichromacy occurs when one of the cone pigments is missing and colour is reduced to two dimensions.

Organisms with dichromacy are called dichromats. Dichromats 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.

Classification
There are various kinds of color blindness:


 * Protanopia is a severe form of red-green color-blindness, where there is impairment in perception of very long wavelengths, such as reds. To these sufferers, reds are "perceived" * as beige and greens tend to "look" * beige like reds.  Protanomaly is a less severe version.
 * Deuteranopia consists of an impairment in perceiving medium wavelengths, such as greens. Deuteranomaly is a less severe form of deuteranopia.  Those living with deuteranomaly cannot see reds and greens like those without this condition; however, they can still distinguish them in most cases.
 * A more rare form of color blindness is tritanopia, where there exists an inability to perceive short wavelengths, such as blues. Sufferers have trouble distinguishing between yellow and blue.  They tend to confuse greens and blues and yellow can "appear" pink.

Animals that are dichromats
It is currently believed that most mammals are dichromats. The straightforward exceptions are primates closely related to humans, which are usually trichromats, and sea mammals (both pinnipeds and cetaceans) which are cone 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 cone monochromats, and both sexes of howler monkeys are trichromats.

Recent research suggests that trichromacy may be widespread among marsupials.

Dichromats are capable of seeing 10,000 different colors
According to color vision researchers at the Medical College of Wisconsin (including Jay Neitz), each of the three standard color-detecting cones in the retina of trichromats – blue, green and red – can pick up about 100 different gradations of color. Since each detector is independent of the others, the total number of colors discernible by an average human is their product, or about 1 million. Similarly, a dichromat (such as a human with red-green color blindness) would be able to distinguish about 10,000 different colors.