Glossary » color blindness
an inability to distinguish certain color combinations. About 8% of males and 0.5% of females are colorblind in some fashion, so it is common enough to be a highly-significant factor in design. Red-green color blindness is most common, followed by yellow-blue color blindness.
To avoid problems for users with color blindness, make sure that color is never the sole way of distinguishing objects. Never expect users to be able to name colors or recognize a color by name. Use differences in brightness to make colored regions distinct, and test your interfaces in grayscale to confirm that they are still usable.