Not one of the roughly 6,500 living mammal species grows a green coat. The reason is a pigment system stripped down long before the dinosaurs disappeared. Every colour in mammalian hair comes from two molecules. Eumelanin covers brown through black. Pheomelanin covers yellow through rust red. That is the entire palette. The agouti banding of a hare, the rosettes of a jaguar, the winter white of an Arctic fox: all of it is those two polymers switching on and off along a growing shaft. Other vertebrates do not depend on pigment alone. Frogs, lizards and fish stack three chromatophore layers in living skin: - xanthophores holding yellow pteridines and carotenoids - iridophores packed with guanine platelets that scatter short wavelengths - melanophores beneath, absorbing what passes through Blue light scattered upward through a yellow filter reads as green. Birds run the same trick with quasi-ordered spongy keratin inside the feather barb, tuned to a coherent scattering peak and overla...