The real reason is that these genes sit on the x chromosome, which makes it possible for women to be heterozygotes. If one of the receptors is mutated and color-shifted, then women will be homozygous for the wavelengths of two of the three receptors, but heterozygous for the remaining receptor, which gives her two more wavelengths to interpret. Therefore, she will have a four-dimensional color space. A male with the same mutation does not have a normal copy of the gene, so still only sees three-dimensional color, with the third dimension different from what most of the rest of us see, which manifests as a type of color blindness, since the rest of society has engineered the world for normal RGB people.