No mammal has ever evolved green fur. Not one of roughly 6,400 species. Mammals make exactly two pigments, eumelanin for black and brown, pheomelanin for red and orange, and no combination of those gets you to green. The only mammal that looks green is a sloth, and that's algae growing on it. So evolution solved the camouflage problem from the opposite end. Deer and boar are dichromats. They lack the long-wavelength cone, so orange and green land on the same channel. A tiger never needed green fur, because the animals it hunts cannot tell the two apart. John Fennell's team at Bristol ran this in simulation, and the tiger goes from garish to invisible the moment you remove one cone. Which leaves the question Fennell flagged as still open. Why didn't deer evolve trichromatic vision? Spotting an ambush predator is the whole ballgame, and primates pulled off that upgrade tens of millions of years ago. The answer is stranger than growing better eyes. In Kanha and Mudumalai, chital deer spend their days directly underneath langur troops. Langurs are Old World monkeys, which means full trichromatic vision, sitting in the canopy, looking down at the forest floor. A 1989 study at Kanha found that of deer herds within 200 meters of a langur troop, 70% closed to within 25 meters. Chital reacted to langur alarm calls more often than langurs reacted to theirs. The deer could not build the eyes. So they parked themselves under something that already had them. The tiger's coat is tuned against the visual system of nearly every mammal in that forest. It fails against exactly one, and the deer worked out how to borrow it. The borrowing works well enough that people run the same play. When a burst of chital alarm calls goes up in an Indian reserve, every guide within earshot stops the jeep and starts looking for stripes.