Flamingos are not pink; they are, in fact, white. However, they are able to deposit carotenoid pigments in their skin and feathers. The birds obtain these pigments via their natural food, particularly from algae. Flamingos assimilate the carotenoids into so-called canthaxanthin (a red pigment also used in the food industry, for example added to sausages) and incorporate that into their plumage.
The surplus of red pigment is stored in the liver and will be used during the next moult to colour the new feathers pink. Flamingos that lack carotenoids in their food will turn white after the next moult, as zookeepers in the past will have witnessed.
Hein van Grouw, Bird Curator, National History Museum