Color vision deficiency is the inability to distinguish certain shades of color. The term "color blindness" is also used to describe this visual condition, but very few people are completely color blind. Most people with color vision deficiency can see colors, but they have difficulty differentiating between particular shades of reds and greens (most common), blues and yellows (less common).

Color vision deficiency can range from mild to severe, depending on the cause. It affects both eyes if it is inherited and usually just one if it is caused by injury or illness. Color vision is possible due to photoreceptors in the retina of the eye known as cones. These cones have light-sensitive pigments that enable us to recognize color. Found in the macula (the central part of the retina), each cone is sensitive to either red, green or blue light. The cones recognize these lights based on their wavelengths.

Normally, the pigments inside the cones register different colors and send that information through the optic nerve to the brain. This enables you to distinguish countless shades of color. But if the cones don't have one or more light-sensitive pigments, you will be unable to see one or more of the three primary colors. The most common form of color deficiency is red-green. This does not mean that people with this deficiency cannot see these colors at all. They simply have a harder time differentiating between them, which can depend on the darkness or lightness of the colors.

What causes color vision deficiency?
Usually, color deficiency is an inherited condition caused by a common X-linked recessive gene, which is passed from a mother to her son. But disease or injury that damages the optic nerve or retina can also cause loss of color recognition. Some diseases that can cause color deficits are:
Diabetes
Glaucoma
Optic Atrophy
Macular degeneration
Alzheimer's disease
Parkinson's disease
Multiple Sclerosis
Chronic alcoholism
Leukemia
 
How is color vision deficiency treated?
There is no cure for inherited color deficiency. But if the cause is an illness or eye injury, treating these conditions may improve color vision. Using special tinted eyeglasses or wearing a red-tinted contact lens on one eye can increase some people's ability to differentiate between colors, though nothing can make them truly see the deficient color.

Most people with color vision deficiency find ways to work around the inability to see certain colors by:
Organizing and labeling clothing, furniture or other colored objects (with the help of friends or family) for ease of recognition.
Remembering the order of things rather than their color. For example a traffic light has red on top, yellow in the middle and green on the bottom.