Color blindness, (also known as Dyschromatopsia) or color vision deficiency, in humans is the inability to perceive differences between some or all colors that other people can distinguish.
The most frequent forms of human color blindness result from problems with either the middle or long wavelength sensitive cone systems, and involve difficulties in discriminating reds, yellows, and greens from one another. They are collectively referred to as "red-green color blindness", though the term is an over-simplification and somewhat misleading. Other forms of color blindness are much more rare. They include problems in discriminating blues from yellows, and the rarest forms of all, complete color blindness or monochromacy, where one cannot distinguish any color from grey, as in a black-and-white movie or photograph.
Color blindness is not the swapping of colors in the observer's eyes. Grass is never red, and stop signs are never green. The color impaired do not learn to call red "green" and vice versa. However, dichromats often confuse red and green items. For example, they may find it difficult to distinguish a Braeburn from a Granny Smith and in some cases, the red and green of a traffic light without other clues (e.g., shape or location).
Color blindness almost never means complete monochromatism. In almost all cases, color blind people retain blue-yellow discrimination, and most color blind individuals are anomalous trichromats rather than complete dichromats. In practice this means that they often retain a limited discrimination along the red-green axis of color space although their ability to separate colors in this dimension is severely reduced.
It should also be noted that even though some people are unable to see some or maybe even any of the numbers in (e.g. red-green) color blindness test, the person might still be able to tell the difference between the colors in his or her everyday life.
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