One of the characteristics by which mammals are defined is the possession of hair, and in most mammals it is their hair which gives them their colour. It is probably that the genes in the earliest mammals from which all others descend did not have the capability for green hair, and there has never been a good reason for any branch of mammals to evolve this particular trait. Green in plants is chlorophyll, which animals do not have. While there green is certainly in the reptile palette of colors, it would appear that the early mammals dropped this long ago. Some sloths have large enough colonies of algae in their fur to give it a greenish tinge.

Why did green skin or fur not evolve in mammels? There are many good readsons to develop camuflage, and green seems like a very good camuflage. Green is a feature of some feathers, and show how it might be done, and thus it mark that it is in theory possible and feasible to do so. One exception might be if green is only made possible in eactly feathers, e.g. because only the physical structure of feathers has the properties requires to reflect the green part of the spectre. Black, brown gray pigments in feathers come from blood. Red, yellow from fat. Lipochrome pigments produce red, orange. yellow. Melanin produces black, brown, red-brown, gray. Blue shimmering due to both pigments and structural colours whivh is a differential scattering of light. Particles within feather scatter short wavelengths. Sources say that its scattering intensity is proportional to the inverse fourth power of the wavelength. Also microscopic ridges on the feathers diffract grating breaking light falling on them into all the colours of the light spectrum. Evolution of camuflage is certainly still working in the realm of mammels. How fundamental are the reason(s) that fur does not mutate and retain the colour green? Since it's not simply one phenomenon that creates all the colours, it is possible that it is highly unlikely that fur should develop green any time soon, simply because it does not presently exhibit any properties that easily create that phenomenon. I could imagine that a green cat population would have been created if cat owners were ever given the chance to make one. Genes from jellyfish have been transplanted into mise and pigs to make them glow with a greenish tint under a black light. But a clear visible green color has not yet been made. Besides, I don't presently believe that genetic manipulation should readily be counted as an integrate part of evolution. Although, I do see that such an argument could be defended.