Color blindness – also called color vision deficiency – is fairly prevalent. It is usually of genetic origin and, depending on the type of color deficiency, occurs in approximately 1% to 8% of males and much fewer females. As I say in the book:
If you create a map for your work‑group, and your work‑group consists of three Caucasian men, the chance that one of them is red-green color deficient is about 22 percent.
One way to make your colors distinguishable is to simply ensure that each color has maximum brightness (in the HSV system it would correspond to the value). A color deficient individual will see the colors differently than others, but will be able to understand the color coding none-the-less. Along these same lines, you can make certain that the colors are not pure. For example you want to avoid pure red: 255, 0, 0 next to pure green: 0, 255, 0 (RGB).
You can double check your final product for color deficient readability using a tool such as Vischeck or Color Oracle. You can also read more – and see some map examples – in this great article: Be Kind to the Color Blind.
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