Many people have seen birds that should be colorful, but are instead completely white, mostly white or have patchy white spots where colorful feathers should be.  Other birds that should be colorful appear to be very darkly colored or with dark feathers where lighter ones should be.  These birds all have a genetic mutation which is expressed either in just their feathers or in all parts of their body.

Pure white birds are Albino.  They have a genetic mutation where they do not produce melanin, the pigment that causes color in skin, feathers, scales and the iris of the eyes.  This mutation also occurs in humans and other animals.  The absence of pigment in the iris causes the eyes to appear red, as all you see are the blood vessels that feed the iris.

Leucistic birds are able to produce melanin, but the color is not expressed in some or all of their feathers.  Pigment is usually expressed in their irises, however, so their eyes are usually colored, as are their feet and bills.  This is the easiest way to tell if a bird is albino or leucistic.  Some leucistic birds are called "piebald" or "pied" when they have a feather pattern that looks like a patchwork quilt.  Marilyn Meadows photographed a leucistic "piebald" Black-Chinned Hummingbird that was coming to one of her feeders.