http://en.wikipedia.org/wiki/Egg_%28food%29
Shell and its colour
Egg shell color is caused by pigment deposition during egg formation in the oviduct and can vary according to breed, from the more common white or brown to pink or speckled blue-green. Although there is no significant link between shell color and nutritional value, there is often a cultural preference for one color over another. For example, in most regions of the United States, eggs are generally white; while in the northeast of that country and in the United Kingdom, eggs are generally light-brown. In Brazil and Poland, white eggs are generally regarded as industrial, and brown or reddish eggs are preferred. Regarding chicken eggs, the color of the egg depends on the breed of the bird. In general, chicken breeds with white ear lobes lay white eggs, whereas chickens with red ear lobes lay brown eggs.