A boiled egg should ordinarily show only two colors: the rich yellow of its yolk and the clean, firm cooked whites. All too often, however, you see a third color, a dismal layer of gray or gray-green surrounding the cooked yolk. It doesn't impair the egg's edibility, but it does take away from its appearance. This gray layer is a sign that the egg was overcooked, which created an unwanted chemical reaction.
Under the Hood
The gray layer is caused by a reaction between the iron in the egg yolks and sulfur in the whites. When the eggs are cooked too long or at too high a temperature, they form a drab compound called "ferrous sulfide." You might also notice a distinct sulfury odor when you peel the eggs. To avoid this reaction, cook the eggs gently at a low temperature. The American Egg Board recommends covering the eggs by at least 1 inch with cold water, and bringing them barely to a boil. As soon as they boil, remove the pot from its burner and cover it. After 12 minutes, large eggs will be perfectly -- and gently -- hard boiled.