While no two individuals can be expected to have exactly the same color experience at any given time when viewing a specific scene, as many as 10% of human males, and .05% of females, perceive colors much differently than the average population. These differences are called color vision deficiencies, and can range from a slight shift from the norm in color perception to perceiving no colors at all, or only perceiving gray values. Visual tests such as the one depicted here are used to evaluate color deficiencies.

Color deficiencies in humans are relatively rare, and most affect males. This is because inherited color perception is passed genetically through the X chromosome, and normal females have two X chromosomes, so the chances of both carrying a color deficiency is rare. Males, however, have a greater chance of inheriting a color deficiency through their single X chromosome, which is passed from the mother.

An estimated 5% of males are anomalous trichromats, those who can see all colors but have deficiencies in one or more photopigments, causing color perception to shift away from the norm.

A dichromat lacks one of the 3 cone photopigments, visually experiencing the world without one of the red, green, or blue stimulations, or with it markedly shifted from the average population. A dichromat lacking erythrolabe, or the photopigment to see red, is known as a protanope or an individual having protanopia. One who lacks chlorolabe, the green photopigment, is called a deuteranope and is affected by deuteranopia. These red/green deficiencies are the most common dichromatic deficiencies, and are usually inherited, affecting about 2% of males and .4% of females.

While it's difficult to know how others with color deficiencies see the world, the images below show the same scene (of red cherries in green leaves) with red colors removed (left) and with green colors removed (right).

The rarest dichromat is the tritanope, who lacks cyanolabe, or the photopigment to receive the color blue. This deficiency, known as tritanopia, is more often acquired than inherited, sometimes by disease or injury. Tritanopia affects less than .1% of the population. A tritan shift can also be caused by cataracts, or yellowing of the cornea caused by exposure to UV rays or by aging.

The most extreme example of color deficiency is the monochromat, who sees the world with only one photopigment. An achromat, also known as a rod monochromat, perceives differences between light and dark gray values but does not see colors. A cone monochromat sees with only one set of cones, perceiving images only in red, green, or blue ranges. Monochromacy is rare, affecting less than .01 % of the population.

We all perceive color differently, but a small percentage of us perceive it markedly differently than most. However, since we are all only aware of own color perception, it's difficult to describe how one person sees color versus another. And many people with color deficiencies have leaned to identify colors using other cues, such as relative lightness and darkness values, combined with shapes of items as when comparing a banana and a tomato. The next modules in this program can help in designing for everyone, in a way that doesn't include identical values that are directly aligned with one another.

Click Next to take the Biology of Color Quiz!