Normal persons are able to match all colors of the spectrum by mixtures of only three fundamental color sensitivities. Hence, the huge variety of colors we perceive stems from the cone cells' response to different compositions of wavelengths of light. Defects in color vision occur when one of the three cone cell color coding structures fails to function properly. One of the visual pigments may be present and functioning abnormally, or it may be absent altogether. For practical purposes, all color-deficient individuals have varieties of red or green deficiency.
Blue deficiencies are very rare.
Color deficient patients are not completely red or green blind. Compared to persons with normal color vision, they have some trouble differentiating between certain colors, but the severity of the color deficiency is variable. Color blindness is normally diagnosed through clinical testing. Although there is no treatment for color blindness, most color deficient persons compensate well for their defect and may even discover instances in which they can discern details and images that would escape normal-sighted persons. The retina is made up of what are called Rods and Cones.