From an Americanwesite - flamingo chicks are born with gray feathers. Why gray? Well, that's a built-in protection mechanism known as camouflage. You see, with gray feathers, the young flamingo is able to blend in perfectly with... well, gray stuff.
Anyway, the flamingo's feathers turn pink as the bird matures. This is caused by chemicals in the shrimp and plankton which they ingest. If you wonder why we humans don't turn pink, it's because we are, for the most part, featherless (also we don't eat as much plankton).