Common grackles display a cool amount of iridescence (that blue-green sheen). This blue-green sheen is not due to the pigmentation of the feathers, but rather is due to how light reflects of it. Different angles of light causes different colors to be reflected off and thus seen. This is why on a cloudy day, iridescence is hard to be seen.
This type of coloration is called structural coloration. It is responsible for the blues, greens and iridescence in all animals.
This is in contrast to biological pigmentation which are reds, browns, and black colors seen in animals. These pigments are caused by organic chemicals that the animal ingests or synthesizes. An example of caroteniod pigments are the red that house finch or northern cardinal produces.
If they ingest less carotenoid containing foods, they will not be as red.
Also, the organic compound melanin, is produced in humans and gives us the color of seen in our skin. Sun tanning results in the synthesis of more melanin and thus darker skin. Melanin is also responsible for black and brown colors found in animals.