The area at the back of the eye, called the retina, is sensitive to light and colour. It contains specialized cells, called cones, which respond to colour. There are three types of cone cells. One responds best to red light, one to green light, and one to blue light. When a specific type of cone cell is missing or doesn't work properly, a person will have trouble seeing the colour that particular cone cell responds to. For example, a person with red colour blindness has a defect in red cone cells.

Most colour blindness is inherited, although some cases are caused by an injury or disease of the retina or optic nerve, the nerve that takes visual information from the eye to the brain. People inherit colour blindness as a result of a defect on the gene(s) for colour located on the X chromosome.

Men inherit colour blindness 10 times as often as women do. Colour blindness "shows up" in men because they have only one X chromosome. Since women inherit two X chromosomes, a healthy gene on one X chromosome can override the unhealthy gene on the other. A woman can still have the unhealthy gene; it just doesn't always show up. She can, however, pass the gene to her children. A person who doesn't have a genetic condition like colour blindness but who can pass it to her children is called a "carrier."

Diseases that may lead to colour blindness include glaucoma, macular degeneration, cataracts, uncontrolled diabetes, and multiple sclerosis. Fading color vision may also occur with age.

Colour blindness ranges from very mild to very severe forms, with most people having mild symptoms. People with colour blindness can't see the difference between certain colours. For example, a person with severe green colour blindness (deuteranopia) has trouble seeing the difference between oranges, greens, browns, and pale reds. In someone with severe red colour blindness (protanopia), all red colours look very dull.

A few people have trouble distinguishing blue. This condition, called tritanopia, is either inherited or is caused by a reaction to drugs or poisons that damage the retina or optic nerve. It can also be due to a loss of function in these two areas over time.

Most people don't know they have colour blindness until someone else notices they have trouble telling shades apart. For example, someone may notice the colour blind person has trouble matching colours.

Tests for colour blindness are generally given to children and to people applying for jobs where colour discrimination is important, such as in the case of pilots, train engineers, or electricians.

Colour blindness is tested in daylight, using special coloured cards. A more complicated test uses an instrument called an anomaloscope. It shines a changing mixture of red and green light. The person is asked to change the mixture until it looks the same as a yellow light. The examiner can tell how severely colour blind a person is by looking at the redness or greenness of the adjusted mixture.

Inherited colour blindness is not treatable. In cases of acquired colour blindness, a doctor will treat the underlying disease or injury. People with mild colour blindness lead fairly normal lives. Severe colour blindness can interfere with tasks such as seeing traffic signals properly. Special eye glasses or contacts may be prescribed to adjust colour. People with severe colour blindness may not be able to perform tasks that require colour discrimination.

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