There are six species of flamingos, four in western hemisphere (Caribbean and South America) and two in Africa and Asia. All are varying shades of pink, but when seen together in the same lake can be difficult to tell apart.
The Greater and Lesser Flamingo often occur together in dense aggregates in the same soda lakes in Africa. As their name suggests, they should be recognized by their size: Greater Flamingos are the largest of the six species; Lesser Flamingos are the smallest of the six. However, the size factor wasn’t working for me when I saw the two species in a small lake at the Safari West wildlife preserve.
American Flamingos are easily distinguished from these two with their bright pink-orange coloration.
The pink color of all flamingos comes directly from their diet and is most likely attributed to a bacteria (blue-green algae actually) that grows in the soda lakes favored by flamingos. Birds that ingest the algae directly are usually darker colored than those that get the same carotenoid pigment from animal elements of their diet, like brine shrimp who also ingest the blue-green algae.