Egg shell and yolk color may vary. Color has no relationship to egg quality, flavor, nutritive value, cooking characteristics or shell thickness.
Shell color comes from pigments in the outer layer of the shell and, in eggs from various commercial breeds, may range from white to deep brown. The breed of hen determines the color of the shell. Among commercial breeds, hens with white feathers and ear lobes lay white-shelled eggs; hens with red feathers and ear lobes lay brown eggs.
White eggs are most in demand among American buyers. In some parts of the country, however, particularly in New England, brown shells are preferred. Commercial brown-egg layers are hens derived from the Rhode Island Red, New Hampshire and Plymouth Rock breeds. Since brown-egg layers are slightly larger birds and require more food, brown eggs are usually more expensive than white.
Egg albumen in raw eggs is opalescent and doesn’t appear white until you beat or cook it.
Yolk color depends on the hen’s diet. If a hen eats plenty of yellow-orange plant pigments called xanthophylls, the xanthophylls will be deposited in the egg yolk. Hens fed mashes containing yellow corn or alfalfa meal lay eggs with medium yellow yolks, while those eating wheat or barley yield lighter-colored yolks. A colorless diet, such as white cornmeal, produces almost colorless yolks. Natural yellow-orange substances, such as marigold petals, may be added to light-colored feeds to enhance yolk color. Artificial color additives are not permitted. Most buyers in this country prefer gold or lemon-colored yolks. Yolk pigments are relatively stable and are not lost or changed in cooking.
A green ring around hard-boiled egg yolks is the result of sulfur and iron compounds in the egg reacting at the surface of the yolk. The greenish color may occur when you overcook eggs or when there is a high amount of iron in the cooking water. Although the color may be unappealing, eggs with green rings are still wholesome and nutritious and have a normal flavor. The best ways to avoid greenish yolks are to use the proper cooking time and temperature and to rapidly cool the cooked eggs.
– See Cooking Methods, Hard-Boiled
Sometimes a large batch of scrambled eggs turns green. Although not pretty, the color change is harmless. Just as in hard-boiled eggs, the green color is the result of heat causing a chemical reaction between the eggs’ iron and sulfur. The green color occurs when you cook eggs at too high a temperature, hold them for too long, or both. To prevent the coloring, use stainless steel equipment and a low cooking temperature, cook the eggs in small batches and serve them as soon as possible after cooking. If it’s necessary to hold scrambled eggs for a short time before serving, it helps to avoid direct heat. Place a pan of hot water between the pan of eggs and the heat source.
Occasionally several concentric green rings appear in hard-boiled egg yolks. A yolk develops within the hen in rings. As the rings are formed, iron in the hen’s feed or water may cause the green coloring.