More accurately called color vision deficiency, color blindness is the inability to distinguish colors, most often shades of red and green. Only rarely does a person not see any color at all (complete achromatopsia). Some people don’t know they see colors incorrectly unless they happen to be tested.
What happens in a person with color vision deficiency? The retina has three types of color-sensing cone cells: red, green, and blue. Normally, signals from cone cells are blended, so the brain perceives a full spectrum of colors. But a genetic defect can cause red to look brown or create confusion between green and orange. Rarely are blue cone cells affected.
Color vision deficiency is almost always hereditary. More men than women (8 percent versus less than 1 percent) are affected, because the faulty gene is carried on the X chromosome. Since men have only one X chromosome, the trait is more likely to be expressed, whereas women, with two X chromosomes, are likely to have a normal gene that will be dominant.
Infrequently, color vision deficiency is a result of retinal disease or some other eye disorder, or a side effect of certain medications.
There’s no cure for inherited color vision deficiency. Some people learn to distinguish colors in other ways (standardized traffic lights, for example, are green on the bottom, red on top). Certain tinted glasses may help in some cases.