http://www.post-gazette.com/pg/06256/721190-114.stm
There is now a developing body of evidence that the human population contains a third, yellow, version of this X chromosome pigment gene. Since there are only two positions on the X chromosome and since males only have one X chromosome, males can only be tri-chromatic. But females, with two X chromosomes having a total of four possible genes, can possess all three versions of the gene. Such females would be tetra-chromatic and would perceive color, especially in the yellow region, differently than tri-chromatic humans.