Flamingos are only pink because of what they eat

Flamingos are born grey and only develop their famous pink-to-red colouration from carotenoid pigments in the algae and shrimp they filter from water. Flamingos kept in captivity turn white if their diet lacks carotenoids — zoos must add these pigments to their feed. The intensity of a flamingo's colour signals its health and nutrition, so the most vividly pink individuals are the most successful at attracting mates.

Source: Hudon, J. et al., 'Diet explains red colouration in flamingos', Comparative Biochemistry and Physiology (2003)