Why do some hard-cooked eggs have a light grey colour on the outside surface of the yolk?
The light grey or greenish colour which forms on the outside surface of the yolk is the result of the reaction between sulfur and iron compounds in eggs. It tends to occur when the eggs are overcooked or when there is a high amount of iron in the cooking water. Although the colour is not very attractive, the eggs are still safe to eat. Their nutrient content and their flavour are unaffected.
