Map Makers Can Avoid Confusing The Color Blind

"Rainbow schemes are very popular in maps designed to convey information," says Cynthia A. Brewer, associate professor of geography. "Unfortunately, rainbow schemes are not necessarily the best way to present information to most color-blind individuals."

Color blindness is not a single problem but a variety of problems that affect color vision. Only a very small portion of people do not see color at all, but view the world in shades of gray. The most common type of color blindness is red/green, which has two forms. Some people are missing red receptor cones and others are missing green receptor cones. People with different forms of red/green color blindness have slight differences in the way they perceive color. Another form of color blindness is blue/yellow, which is less common. While only 8 percent of men are color-blind, they make up 95 percent of the 9,000,000 people in the U.S. who suffer color-vision impairment.

"I am only attempting to accommodate those who are red/green color blind because combined they are the most common forms of color blindness," says Brewer. "It would be a different and more difficult task to accommodate everyone on the same map."

The reason one solution will not work for all types of color blindness is that color confusion occurs along lines through the full color space enclosed by the spectrum. Different sets of confusion lines are specific for the type of color blindness. Luckily, the confusion lines for both types of red/green color blindness, while different, are similar enough that colors can be chosen to serve both sets of individuals.

Red/green color blindness is not simply a problem with confusing red and green. It also causes problems with an unlimited pairing of colors that fall on the confusion line. For example problems can occur distinguishing between blue-green and pink or blue-green and purple. Color-blind individuals may not be able to distinguish between olive-colored and rust-colored socks, while they could distinguish between bright green and olive socks, rust and red socks or rust and bright green socks.

One way to avoid confusion is to alter the lightness and darkness of the colors. The color-blind person may still see the same color, but they can tell that the areas colored with these colors are different.

"We do not know what color-blind people see," says Brewer. "Actually, we do not know what color vision looks like to anyone but ourselves. However, if a map has adjacent patches that someone sees as the same color, the information stored in the map will be inaccessible."

Brewer began working on mapping for the color blind as an assistant to Dr. Judy Olson at Michigan State University. More recently, Brewer had an opportunity to design red/green color-blind friendly color schemes for the Atlas of U.S. Mortality produced by the National Center for Health Statistics. These epidemiological maps contain large amounts of information that do provide a thousand words in one picture, if the viewer can see the picture as it is intended.

One way to accommodate red/green color blindness and still use a rainbow scheme is simply to skip over one of the offending colors. "If we use a scheme that goes red to blue and leaves out the greens then it is not a problem," says Brewer.

Other successful color combinations include blues and yellows, magenta-violets and yellow-reds and blue, green, yellow sequences. Again, combining alterations in color purity and hue also help color-blind people to distinguish between colors.

The key in choosing color schemes is to choose colors that do not lie on confusion lines. For example, in a color scheme using blue, magenta, green, red-orange and yellow, the last three colors fall on the same confusion line and would cause problems. A solution would be to choose blue, light green, greenish white, gray magenta and dark green, a scheme that avoids confusion lines.

Cartographers typically spend time designing demographic maps for deaths, births, populations, agricultural and industrial production and disease. If they consider the color blind when making their decisions and choose colors that do not lie on the same confusion line, the information they are trying to communicate will be accessible to nearly anyone who sees it.