Actually red fades the fastest. Red has the longest wavelength of visible colors which causes it to fade quicker than others. Generally the higher wavelength the absorbed light, the more likely it is to cause a pigment breakdown. This is because the energy of each photon in light is proportional to the wavelength (Energy of a photon = Planck's constant times wavelength). Blue dyes reflect higher visible wavelengths (not absorb) so are therefore likely to have a minor fade resistance advantage over other colors. The science of it is even more complicated than that and way over my head but this is the cliff note version.