According to Wiki, it's a combination of optical and biologial effect.
"Many of the Hummingbird species have bright plumage with exotic colouration. In many species, the coloring does not come from pigmentation in the feather structure, but instead from prism-like cells within the top layers of the feathers. When light hits these cells, it is split into wavelengths that reflect to the observer in varying degrees of intensity. The Hummingbird wing structure acts as a diffraction grating. The result is that, merely by shifting position, a muted-looking bird will suddenly become fiery red or vivid green. However, not all hummingbird colors are due to the prism feather structure. The rusty browns of Allen's and Rufous Hummingbirds come from pigmentation. Iridescent hummingbird colors actually result from a combination of refraction and pigmentation, since the diffraction structures themselves are made of melanin, a pigment"
http://en.wikipedia.org/wiki/Hum ... ructure_and_colours
