People who are color-blind can't perceive differences between some of the shades of colors that others can distinguish. The most common test for color-blindness involves showing the viewer a circle comprised of dots. In the middle of the circle, dots of a slightly different color or shade are arranged in the shape of a number. Most people have no problem picking out the number hidden in the dots, but those who are color-blind can't discriminate between the different shades and don't see the number. This test is formally called the "Ishihara Test for Color Blindness," and several variations of it are used to detect the various types of color-blindness.
Color-blindness arises when a person has missing or defective color receptors in his or her eyes. Normally, there are three kinds of color receptors (cones) in the eye: one that detects blue, one that detects green, and one that detects red/yellow. The sensitivity of normal color vision depends on the overlap of signals from these three types of color receptors. Different colors and their shades are recognized when the three types of cones are stimulated to differing extents. Functionally, this is similar to the mixing of different colors of paints (e.g., red + blue = purple).
A common misconception is that all color-blind people see the world in gray. In most cases, when someone is color-blind, only one of his three types of color receptors is missing or defective. Thus, people who are color-blind usually only have difficulty distinguishing between red and green or between blue and yellow. In rare circumstances, all three types of cones are missing/defective, and the color-blind person can only see in black-and-white (as if he were watching a 1950s film).
Color-blindness affects 5% to 8% of all males and 0.4% percent of females. The condition afflicts more males than females because most genes for color vision are carried on the X-chromosome. Since only one fully functional copy of a color-receptor gene is required for normal color vision, females, who have two copies of the X-chromosome, have a lower risk of developing color-blindness. (Both of her parents must give her a color vision-defective copy of the X-chromosome for her to be color-blind.) Males on the other hand, who only have one copy of the X-chromosome, need only to receive one defective copy of the X-chromosome (from their mothers) to be color-blind.
Although color-blindness is most often genetic in nature, it may also occur in response to eye, nerve, or brain damage. There is no treatment or cure for color-blindness, but certain types of tinted eyeglasses or contact lenses may help a color-blind person distinguish between different colors better. Although color blindness is often seen as a disability or deficiency that needs to be fixed, several studies have shown that color-blind individuals are actually better than others at certain tasks such as seeing through camouflage.