For once the old man leveled with you. Flamingos are in fact pink because they eat shrimps, which contain a red pigment. I confess this knowledge causes me some distress, because it reinforces the cartoon view of the universe, which I learned at an early age to distrust. In cartoons, exploding bombs merely blacken your face, a character who runs off a cliff can pinwheel in midair for several seconds before plunging earthward, and if you drink a bottle of red ink you'll turn red. Common sense (or painful experience) soon teaches us the truth about shrapnel and gravity. But drinking that red ink? It's honest to God the way the world works.
What we're talking about here is a class of plant-derived compounds called carotenoids--that is, carotene and its chemical cousins. People who are regular Straight Dope readers--heck, people who are regular breathers--are familiar with carotenoids, which have many useful properties, the most notable of which is that carotene is a precursor of vitamin A. Beta carotene, found in so-called cruciform (Latin for “icky”) vegetables such as broccoli and turnip greens, is thought to prevent cancer. More relevant to this discussion is the fact that carotenoids tend to be a persistent red orange in color. Depending on formulation, they impart a red, pink, orange, yellow, or even green cast to some plants and to the higher links in the food chain by which those plants are consumed--and thank goodness, because it would be a dull world otherwise. Examples:
Wild flamingos are pink because they eat carotene-containing brine shrimps (one species) or blue-green algae (another species). Lacking such food in zoos, they turn white unless fed caroteniferous substitutes--carrots, beets, or red peppers in the old days, or the additive canthaxanthin today.
Salmon caught in the wild are orange because of their diet of crustaceans that contain carotenoid. The flesh of farm-raised salmon, which don't feed on crustaceans, is an unappetizing gray unless the fish are given the carotenoid astaxanthin.
Carotenoids are often found in conjunction with chlorophyll, the chemical that makes plants green. When chlorophyll production stops in autumn, the green fades but the carotenoids remain, turning tree leaves red and orange.
Normal human skin color doesn't derive from carotene. However, Straight Dope readers know that (1) if they eat enough carrots (which, duh, contain carotene), they'll turn orange, and (2) if they have no interest in eating vegetables but would still like to be orange, they can take beta carotene pills providing 10 to 30 times the normal dietary amount. This is done in the ill-founded belief, apparently common among northern Europeans, that the resultant color resembles a suntan and will cause people to think, “Ach, Fritz, you look like you chust got back from der Riviera.” On the contrary, you look like you've got hepatitis B.
“The plumage of canaries can be changed from yellow to red if they are fed paprika during molt.” This comes from Straight Dope curator of birds George Angehr, who says he got it from the Cambridge Encyclopedia of Ornithology. Knowing George as a kid would have added a whole new dimension to pet ownership.
George also reports that the “bloodsucking bug Rhodnius as well as several species of tapeworm derive their porphyrin pigments from the heme unit in the hemoglobin of their host.” This has nothing to do with carotene but does indicate that the stuff isn't the only pigment in the world and also gives my assistant Jill the opportunity to remark, “Edith, what are you feeding that tapeworm of yours to give it that healthy glow?”
Nature abounds with other examples of carotenoid-based pigments, which can be found in egg yolks, oranges, butter, tomatoes, mangoes, and many other fruits and vegetables, all of which can be used to generate an array of exciting party colors in your higher vertebrates. But why stop with yellow, orange, pink, and red? I'm thinking of those blue-green algae. A little carotene, a little chlorophyll, and whoa, you're walking into the next sock hop the color of a kelp.