Egg Shell White?

Wild Turkey Eggs

What looks like a white egg shell to us may be an egg of a different color to a bird.  This is because birds see more color than we do.  According to livescience.com, ultraviolet pigments that are invisible to us may help birds distinguish between their eggs and those of another species.

"Unlike humans, birds can see ultraviolet (UV) light, and they have four – rather than three – color receptors in their eyes, allowing them to better distinguish between hues."

While we can recognized many of the differences between different eggs, we can't recognize the UV spectrum of short wavelength light seen by birds.  Nest parasites such as cowbirds and cuckoos lay their eggs in other birds nests.  Unsuspecting birds may nurture these eggs to the detriment of their own eggs.  Scientists suspect that the differences in UV colors play a part in the recognition and rejection of  these foreign eggs.
The article discusses the coloration of egg shells by two pigments and the mysteries of how and why these developed.  The basic egg is white from its calcium carbonate.  Any color which appears, solid or speckled, comes from a mixture of "blue-green biliverdin, and red-brown protoporphyrin, which are both breakdown products of hemoglobin."

While different colored eggs allows species to detect their own eggs, the colors also may serve to camouflage eggs from predators, act as a sun block or actually strengthen the shell.  Even a single bird's eggs may vary in color as the last eggs may have more speckles as the supply of calcium for egg shells is depleted.  Wikipedia on egg biology

Mysteries remain.  We are all familiar with the beautiful powder blue eggs of bluebirds, yet 5% of nestings produce all white eggs.  At times there is a slight pink or bluish tinge to the eggs but all the eggs will be the same color and produce normal colored chicks.  Apparently a bird will always produce the same color eggs.  If a egg of another color appears in the clutch it was dumped by another bird. 

Bluebird eggs- white

The biology and genetics of bluebird egg color is still in debate.  Some birds hatched out of white eggs subsequently lay blue eggs.  Sialis.org  has a lot more information on the subject.