Birds are among the most colorful of all vertebrates. Only coral reef fish of tropical oceans show as wide a range of colors and patterns. Bird feathers show color in two ways. The first and most obvious way is by pigmentation. Like paints, pigments are chemical compounds that absorb certain wavelengths of light while reflecting others. The colors you see are those reflected back. The principal pigments found in bird feathers are of 3 types. Melanins, the most common pigments produce grays, blacks, and browns. Carotenoids, produce intense red of the male Northern Cardinal and the bright yellow of the male American Goldfinch. A third class of pigments, porphyrins, produce a range of reds, browns, and some greens, notably the intense red and greens of African turacos and the brown pigments of the many owls.
Some greens, all blues, and whites are produced by the prism effect, known as structural coloration. The colors are created when light enters a feather and is refracted and bounced through a complex system of cell surfaces within the feather barbs. A Blue Jay's feathers are not really blue at all. If you get a chance to look at one with the light coming from behind it instead of in front of it, you will see this. But when light enters one of its feathers from the outer surface, it gets refracted to show the blue. Therefore, we see the Blue Jay as being blue.
Iridescence is another trick of light. We see it as a shimmering of color on some birds' feathers. Notable examples of this are the red throat feathers (gorget) of the Ruby-throated Hummingbird. Light enters these feathers and interacts with the cell membranes as described for the Blue Jay, but also with a black pigment called melanin. If you watch a hummingbird long enough, you may notice that its throat feathers alternate between the shimmering ruby-red and black, depending on your angle of view. When you see the gorget in the light, it appears red. When it is in the shadow, it appears black - the color of the melanin granules embedded within the feathers.