Two explanations for green color in terrestrial animals that come to mind are (1) camouflage and (2) sexual selection for bright external colors. Many mammals

Two explanations for green color in terrestrial animals that come to mind are (1) camouflage and (2) sexual selection for bright external colors. Many mammals are nocturnal, which renders both of those inapplicable. But, one might think that green would be advantageous as camouflage to arboreal diurnal mammals such as monkeys, that have predators including large forest hawks and eagles. However, just because you have a selective hypothesis doesn't mean you necessarily expect it to have occurred in evolutionary history. In particular, we may not be looking at equilibrium states: perhaps the monkeys were nocturnal until recently, or perhaps vegetation distribution shifts have recently brought the predator-prey combinations we see today into contact.