Color Blindness - National Library of Medicine - PubMed Health
About the Cone Cells
About Color Blindness
Most of us share a common color vision sensory experience.  Some people, however, have a color vision deficiency, which means their perception of colors is different from what most of us see.  The most severe forms of these deficiencies are referred to as color blindness. People with color blindness aren't aware of differences among colors that are obvious to the rest of us.  People who don't have the more severe types of color blindness may not even be aware of their condition unless they're tested in a clinic or laboratory.
Inherited color blindness is caused by abnormal photopigments. These color-detecting molecules are located in cone-shaped cells within the retina , called cone cells . In humans, several genes are needed for the body to make photopigments, and defects in these genes can lead to color blindness.
There are three main kinds of color blindness, based on photopigment defects in the three different kinds of cones that respond to blue, green, and red light.  Red-green color blindness is the most common, followed by blue-green color blindness. A complete absence of color vision —total color blindness — is rare.
Sometimes color blindness can be caused by physical or chemical damage to the eye, the optic nerve, or parts of the brain that process color information. Color vision can also decline with age, most often because of cataract - a clouding and yellowing of the eye's lens. NIH - National Eye Institute
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