Birds use the color (and shape) of plumage as a way to identify males and females of their own species. That's one of the reasons why birds are so various.
There are two seagulls, the glaucous gull (Larus hyperboreus) and Thayer's gull (Larus thayeri) which are very similar. However, they do not breed together in the wild.
The glaucous gull has a yellowish eye ring, and Thayer's Gull has a purple eye ring. A researcher painted the eye rings of Thayer's males yellow, and this allowed Thayer males to successfully pair with glaucous females. However, the males would not mate with the glaucous females until the researcher painted the glaucous females purple.