I’m curious whether anyone can convincingly explain why green mammals or blue mammals have not evolved. (On earth, I mean.) We have green and blue fish, birds, reptiles, amphibians and insects, so why not mammals? I know bright coloration exists for varying reasons, but surely some of these would have been beneficial for some mammals somewhere?
Not exactly what you are asking, but most sloths are covered with algae which gives them a greenish tinge and supposedly camouflages them.
Mammals aside from some primates are effectively red-green colorblind, so any green coloration for non-camouflage purposes isn’t terribly useful.
We don’t actually have green or blue birds, at least not in the sense of having green or blue pigments. Blue feathers in birds are a result of scattering processes in transparent feather structures, similar to what causes blue skies or blue eyes, and green is a combination of such effects with yellow pigments. If you take blue bird feathers, grind them up, and make paint out of them, you’ll get white paint, and if you do the same with green feathers, you’ll get yellow paint.
I think the same is true of insects, but I’m not sure what accounts for green reptiles and amphibians.
Some whales are a bit blue-ish.
I recall Cecil dealing with this question specifically in respect to animals (the idea being that in some environments, green hair/fur would be advantageous for concealment), but I haven’t found it. My recollection of the answer is that, in a nutshell, it’s because the genes that would produce it haven’t arisen.
I’ve seen greenish furred monkeys and also monkeys with bright blue testicals. BRIGHT blue…lots of fun to be had watching moms at the zoo trying to explain 'em!
You mean this column. And as that column says, there is one bird, the turaco, which has true blue or green pigments.
A photo here, without the vitals though. I don’t recall testicles as such, my mom must have been convincing.
It would be very useful if you were subject to predation by birds, as many small mammals are. Most birds have excellent color vision, better than ours. The mammal would not have to be able to see its own color for it to be useful camouflage.
Come to that, who are the sloths supposed to be hiding from by being green? THey are too big for birds, surely. Monkeys? Snakes maybe? (I am not sure about color vision in reptiles.)
Are they necessarily hiding, or is the algae just along for the ride? The relationship doesn’t have to actually help the sloth, as long as it doesn’t hurt it.
Then, too, they might be hiding from us. It doesn’t seem to me that it’d take all that long to evolve something like that, so it’s plausible that it’s a response to hunting by humans.
Do things really “evolve” parasites to help them?
Well, evolve a favorable environment for them, at least. And if they’re helping, then they’re not exactly parasites.
As colibri notes, mammsls can’t actually produce blue or green pigments, so it’s going to be very difficult for them to be blue or green. Birds get away with it because the structure of feathers readily lends itself to diffraction patterning. the same isn’t true of hair.
And that’s the answer in a nutshell. Mammals aren’t green because there is no way for them to be green and still retain hair.
Mammals could have blue skin, and indeed some species do have patches of bright blue skin, but the point ot realise is that it requires the skin to be naked. The only mammals that are naked are the ones that stand to gain little from being blue or green: burrowers, huge cetaceans and pachyderms and top-of-the-food-chain humans.
So really the only way that we could get blue mammals is if one of the already hairless species happened to adopt blue colouration in sexual selection. While that’s certainly possible it;s also essentially random, so you;d need the coincidence of two rather improbable events. Hardly surprising it never happened.
Well, there are those people who drink colloidal silver…
Most mammals are dichromats, thus mostly color blind. A mammal is most likely going to be predated on by a another mammal. Turns out having brown-ish fur is good enough camouflage that there’s not much evolutionary pressure to improve that camouflage.
Not to mention, what “looks” best to humans may not look best to dichromats. If most predators have deuteranopia then they will have a hard time seeing green to begin with, so a brown animal against a green field is still hard to see.
It’s a general truism that you can use evolutionary theory to answer the question “Why?” but not the question “Why not?”
Many see color similarly to the way birds do, but snakes and certain lizards have fewer kinds of photoreceptors and might only see blue and green, or yellow and ultraviolet depending on the species.
There are a lot of green mammals. They are just so well camofloushed with the grass we haven’t found them yet 
The reason I always hear given, since I was a kid in high school is that hair fades quickly and even if it grew in green it wouldn’t stay green for very long. And then it’d fade and the incoming hair would be green and you would have different shades of green and it would make the mammal different colored and more visable.
Same applies to any color and that’s why mammals are so dull colored.
The color of skin is similar but also has other facts like blushing and collagen which accounts for some of the difference.
I don’t really buy it but that is what my high school biology teacher told us when someone asked the question.
We’ve been here a few times before. There are some African monkeys that are described as “green”. In fact the common name for one species of vervet is the “African green monkey”, and the talapoin is sometimes described as “green”. It’s stretching it, though - they’re more grey or brown with a greenish or yellowish tinge. Certainly not bright green.