Any recessive genetic characteristic that persists at a level as high as 5% is generally regarded as having some sort of evolutionary advantage over the long term, such as better discrimination of color camouflaged objects especially in low-light conditions. At one time, the U.S. Army found that color blind people could spot “camouflage” colors that fooled those with normal color vision. It is worth noting, however, that the effect is frequency dependent, because, if the majority of people were dichromats, camouflage dyes would be selected on the basis of deceiving dichromats instead.
Another possible advantage might result from the presence of a tetrachromic female. Owing to X-chromosome inactivation, females who are heterozygous for anomalous trichromacy ought to have at least four types of cone in their retinae. It is possible that this affords them an extra dimension of color vision, by analogy to New World monkeys where heterozygous females gain trichromacy in a basically dichromatic species. Humans have a higher percentage of color blindness than macaque monkeys according to recent research.
