Several views of the same individual male Anna's Hummingbird (Calypte anna) — photographed across from Westside Park on 25 May 2012.
Because the color is dependent on the microscopic structure of the feathers and the refraction of light, it changes with the viewing angle.
From The Birds of North America account by Clark & Russell (2012):
Lesson regarded it as one of the most beautiful hummingbirds, on account of “the bright sparkle of a red cap of the richest amethyst...” on the male’s head, and so named it after the duchess of Rivoli, Anna de Belle Masséna. Gould (1861) placed it in a new genus, Calypte, for “not only the throat, but the entire head as glitteringly resplendent as if they had been dipped in molten metal”.