Why don't mammals use blue or green colouration?

Animals are the colour they are to blend in with their environment. The key thing that drives survival is not being preyed upon or eaten. If you're not eaten then you get to have babies and you pass your genes on to the next generation. If you're a polar bear and your genes make you white, then you're less likely to be caught and eaten. As a result, you're more likely to have babies and those babies will have babies. So animals will change their colour so that they're the same as the surroundings and blend in. That either makes their hunting easier or they don't get eaten by other things. It might be that the colours you're referring to have not been adopted because they're not very good colours in nature. A snake is a greeny-brown colour because it wants to blend into the background. Other animals use the converse. They don't want to blend in because they want to mark out that they're toxic. That's why they use colour. The other reason they use colour, especially in things like fish, is for part of the mating game. It can be used for recognition and communication. Actually, I think baboons have big blue noses and I think there is a mammal that is green, although it's for a slightly different reason. It's called a three-toed sloth. The reason it's green is because moss grows on its fur as it moves so slowly.