My day job is Unemployed. That said, I did work a lot with animals at a Zoo. I also am something of a wild nature boy, with almost half a century of observations of a personal nature. In a fantasy setting, nothing like experience to throw right out the window.
Many creatures in nature are going to have some sort of low contrast coloration to their Normal Environment. With intelligent creatures, this is pretty much moot with clothing, armors, tattoos, etc.
Even so, humanimals, when left to their own devices, tend to do their best to either blend in or stand out, depending on the situation. Most mammalian species (or something similarly distinguishable in this setting) will tend to have natural colorations that help it to blend in against (or within) it's natural setting. I can't tell you how many times people walked up to the Lion Enclosure, with him sitting stock-still, flattened to the ground (a LARGE tawny meat eater with a head the size of a Semi Tire) and stood there for like 4-8 seconds looking all over the cage for it -- and it was literally close enough to have eaten them if there was no cage!
For most animals, this is really the key. Stillness. Either as predator or prey, a low contrast color scheme can make all the difference.
Then you've got those Zebra, white and black zigzags. Best guess? The color pattern (and them being large herd animals) serves to create a series of alternating imagery on the eyes of the predator, making it hard to pick one out of the crowd when they are moving. If you can't tell the butt from the head, it makes selecting a specific prey hard to do.
How does a Grey Elephant blend in to the natural environment? He doesn't have to! Elephants have only one natural predator: Man. Everything else gets the hell out of their way.
Insects, birds and fish: Here there is much more variety. You have camoflage that serves to make them nearly invisible (adaptvie patterning and shape/form) to so dang bright you'd swear you were hallucinating. peacocks? Bright displays with large tail feathers that can fan out and up to do what? Most people think it's nothing more than a courting display -- but no, it's primarily a threat display. The feathers make the peacock loom much larger than it really is. Plus all those eyespots? Makes animals think (hypothetically speaking, no peacock's ever actually divulged the reasoning) that it's looking at them no matter where they are (in front of the creature anyway.)
So apply a bit of that to any animal colorations. Umber Hulks? Should look, both in color and shape of skin texture, pretty rocklike. Owlbears? Combine a bear's brown/black hide/fur with the normally subdued color pattern of one of the single most succesful raptors.
That whole bright color (I'm dangerous!) is also probably accurate. By that token, one might expect a cockatrice to be pretty brightly colored (Like the dilophosuar in Jurassic Park), not only as a warning, but to attract a direct stare. Yeah?
One thing I have noticed in my studies of animals and people is this: Nature abhors uniquity. What I mean by this is simple. Elephants have large ears. Do they help them hear or allow them to cool the blood? Both. Nature likes dual (or more) function in the adaptations animals have.
Zebras. Pattern to create confusion or natural air conditioning? Probably both.
So keep those things in mind when tinting and coloring creatures. Also, with fantasy creatures, it's a good idea to do a couple things: Get a monster manual for one (it usually has a fairly detailed description (in text) of the creatures) and also read up on mythological information about "historical" depictions and descriptions of mythical creatures. Some of what you take for granted about them will surprise you when reading.
So there you go, kamalpoe is right on target about me -- again.
dunniteowl