In most biomes on earth, and certainly in the most densely-populated, the best camouflage color or pattern for an animal would be a variant of green, so as to blend in with the leaves and grasses of the surrounding environment. Whether predator or prey, the art of not being seen is hugely important in the natural world, and yet to the best of my knowledge there isn't a single mammal on the planet with fur that is even partially green.
Is there a reason for that? It seems like animals have evolved some pretty crazy color patterns to break up their silhouette or blend into the background, but why haven't any mammals taken the obvious route of "look like leaves"?
Is it really THAT not-easy being green?