From Harvard Magazine:
The poisonous dart frogs use conspicuous color to tell predators that they are not good to eat. Similarly, a venomous coral snake sports rings of bright color to advertise that it isn’t to be messed with—by a bird considering it for lunch, for instance—while a milk snake, which isn’t poisonous and could be taken quite safely, looks much like a coral snake and trades on the latter’s reputation. Bauer fellow Marcus Kronforst studies a bad-tasting species of butterfly that is orange, black, and yellow, and other species of unsavory butterflies that mimic its color and pattern to form a uniformed corps of the unappetizing. Colors can conceal as well as warn, as in, “You can’t eat me because you can’t see me—I’m a cuttlefish and can change my color to match my background in a millisecond.” Or, conversely, “I’m so well camouflaged with stripes [or spots] that you can’t see me creeping up…and I’m going to eat you.” Hopi Hoekstra, Loeb associate professor of the natural sciences, studies the genetic mechanisms at work in a species of mouse that adapts to its environment by being sand-colored if it lives at the beach and dark if it lives inland.
Perhaps some of the richest language of color has to do with sex. Janis Sacco, director of exhibitions, offers such examples as a male bird of paradise from New Guinea that not only sports much bolder colors than any female of the species, but also a ludicrously long tail. “Pick me as a mate,” he says, “because I have this splendid thing you females may select mates on the basis of, and I must be fit because I have managed to survive despite having to carry it around with me while wearing these obvious colors.”