A person with color blindness has trouble seeing red, green, blue, or mixtures of these colors. The term “color vision problem” is often used instead of color blindness because most people with color blindness can see some color. Very few people with color blindness can see no color at all.

People see color with light-sensitive cells called cone cells, which are located in a special layer of cells at the back of the eye (the retina). Normally, a person has three types of cone cells. Each type is sensitive to either red, green, or blue light. People see color when their cone cells sense different amounts of these three basic colors. A person who has a color vision problem may not see one of these three colors or may see a variation of that color or a different color.

Cone cells are concentrated mostly in the central part of the retina (the macula) to provide clear, sharp color vision. They work best in relatively bright light, so it is difficult to see colors in dim light.

Other light-sensitive cells, called rod cells, sense black, white, and gray shades (light and shadow) but do not see color. Rod cells are very sensitive and function better in dim lighting. However, rod cells are more widely spaced across the retina, so vision with rod cells is less clear and sharp. The eyes rely more on rod cells to see in dim light; this is why a person's vision is not as good in low light.

See an illustration of the eye that shows the retina and the macula.

The severity of color vision problems can vary. In some people the problem is very minor, and they may not even be aware that they see color differently from people with normal color vision. In other people the color vision problem is severe, and they can distinguish only a few shades (a person with normal color vision can see many thousands of colors). In rare cases a person cannot see color at all, and sees only black, white, and gray.

Most color vision problems are inherited (genetic) color vision problems, and are present at birth. Other color vision problems are caused by aging, disease, or injury to the eye. These are called acquired color vision problems. Inherited color vision problems are more common than acquired problems and affect males far more often than females.

Inherited color vision problems cannot be treated or corrected. Some acquired color vision problems can be treated, depending on the cause.

A color vision problem can have a significant effect on a person's life. Color vision problems can affect learning abilities and reading development, and may limit a person's career choices. However, children and adults with color vision problems can learn to make up for their inability to see colors. For more detailed information, see the Home Treatment section.