Reds And Greens
The first study of color blindness was published in 1794 by physicist John Dalton, who was color-deficient himself. The condition Dalton described is not actually any sort of blindness. Color blindness does not affect the overall visual acuity of individuals with the condition. A small number of people can not distinguish between any color and see all things in shades of gray.
People who are color blind often are not aware they have a problem until they are asked to distinguish between reds and greens. This is the most common problem among individuals who are color blind. Some people who are color blind also have trouble telling the difference between green and yellow.
Color blindness stems from a problem in the cone cells of the retina. Light rays enter the eye in some combination of red, green, or blue. Normal cone cells contain light-sensitive molecules sensitive to one of the color spectrum's band of colors. Short-wave cone cells absorb blue, middle-wave cone cells absorb green, and long-wave cone cells absorb red.
Individuals with a color defect do not have a normal complement of these substances, and may be missing one or more of them. Some people who are color blind have trouble distinguishing between reds and greens when the light is dim, but are capable of seeing the difference between the two colors in good light. A less common type of color blindness makes distinguishing between reds and greens difficult regardless of the light quality.
A simple test for color blindness involves the use of cards with dots in different colors. Individuals who are color blind see different numbers or words than those who have a complete range of color vision.