Color Blindness - Problems and Compensations

Problems and Compensations

Color blindness very rarely 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.

Dichromats often confuse red and green items. For example, they may find it difficult to distinguish a Braeburn apple from a Granny Smith and in some cases, the red and green of traffic light without other clues (for example, shape or position). The vision of dichromats may also be compared to images produced by a color printer that has run out of the ink in one of its three color cartridges (for protanopes and deuteranopes, the magenta cartridge, and for tritanopes, the yellow cartridge). Dichromats tend to learn to use texture and shape clues and so are often able to penetrate camouflage that has been designed to deceive individuals with color-normal vision.

Traffic light colors are confusing to some dichromats as there is insufficient apparent difference between the red/amber traffic lights, and that of sodium street lamps; also the green can be confused with a grubby white lamp. This is a risk factor on high-speed undulating roads where angular cues cannot be used. British Rail color lamp signals use more easily identifiable colors: the red is blood red, the amber is yellow and the green is a bluish color. Most British road traffic lights are mounted vertically on a black rectangle with a white border (forming a "sighting board") and so dichromats can look for the position of the light within the rectangle — top, middle or bottom. In the Eastern provinces of Canada horizontally mounted traffic lights are generally differentiated by shape to facilitate identification for those with color blindness.