It’s estimated that about 7% of males are color blind in some form (as compared with 0.4% of females). While a very small percentage don’t see color at all, the most common form of color blindness is the inability to differentiate red and green colors. One area that color blindness can be challenging is in reading and interpreting maps. According to Cynthia A. Brewer, a professor of Geography at Penn State, simply avoiding the juxtaposition of red and green isn’t enough to address the issue of color-blindess:
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.
The Ordnance Survey recently posted about efforts by its cartographers to overhaul the color scheme used with OS VectorMap Local (via the Map Room). Simon Duquénoy, one of the Senior Technical Product Managers worked with a user group made up of Ordnance Survey staff members with various forms of color blindness.
There are tools available online for cartographers to use when attempting to develop color blind friendly maps. Vischeck simulates color blindness and can be run on images of maps or on web sites. Color Oracle is another color blind stimulator cartographers download (runs on Windows, Mac, and Linux). Both will show you processed images of how your map image would look to users that have different forms of color blindness. Colorbrewer has a “colorblind safe” option for picking mapping schemes (Steve Gardner also produced a thesis evaluating Colorbrewer’s color schemes for those with color blindness).