Most people with color blindness can see some color. People who can see no color at all are actually pretty rare!
In a normal eye, there are color-sensitive cells in the retina — the layer of nerves at the back of the eye that convert light into signals sent to the brain. These color-sensitive cells are called cones, and they come in three types: sensitive to red, blue, or green light. In a normal eye, you see color when the cone cells sense any or all of these types of light; cone cells work best in bright light and not as well in dim light. A person with color blindness lacks some of these color-sensitive cells. They may have trouble seeing red, blue, green, or some combination of the three.
Symptoms vary from person to person, and depend on the severity of the color blindness. Some people may not even realize that they see color differently from people with normal color vision! At the more severe end of the spectrum, a person may only be able to see a few shades — a person with normal vision can see thousands of colors. In a few rare cases of color blindness, a person sees only black, white, and shades of grey.
So what causes color blindness? Most color vision problems are inherited, and are present at birth. Less often, color vision problems develop as a result of aging, disease, or injury to the eye. These are known as acquired vision problems, and are more often seen in men than in women.
A simple and common test can measure your ability to recognize colors. It is called the pseudoisochromatic plate test — but you may know it as the test where you look at the colored dots with a number or letter inside. The type of color vision problem you have can quickly and easily be determined by what you can and can’t see in the plates. Another test — called the arrangement test — is often used in acquired vision problems. A person is asked to arrange colored tiles in a specific order according to hue. People who are color blind will have difficulty arranging the tiles correctly.
Unfortunately, genetic color blindness cannot be treated or corrected. Acquired color vision problems may be treated, depending on the cause. For example, if a cataract is causing problems with color vision, surgery to remove the cataract may restore normal vision.