Great answers, and I don't (didn't until now) have the biology behind it, but isn't the environment on which each species habitats a reason for it also? If we see it as a painting, birds "background" is sky (blue) andr trees (green) and colorful flowers so to camouflage they need to be colorful. Same goes for fish: sea (blue) , seaweed (greenish-yeallow), corals colorful. Also fish have this silver reflective effect to mimic sunlight in the sea. And packs of birds white belly black top, so when they Move in hundreds they look like one big bird. Also there are the octopus and other sea creatures but also chameleons that can change color. Insects that look like flowers or sticks. On the contrary, lions and tigers and pumas and other mammals live in Savannah, where the background is mainly ochre yellow from the sun and grass so that is their best camouflage option. The spots on some help with the camouflage also. Mice and castors and the rest have the ground as a "background" so dark grey or dark brown would be the best because they live under the surface. I mean, if I think about zebras, they didn't make the best choice, but at least they move in packs and it's quantity over quality. So isn't firstly the survival the evolution reason, and then the mating?