In a sulfurous episode of Today I Found Out, host Simon Whistler explains the chemical reaction that takes place to cause the yolk of an overcooked hard boiled egg to turn an unpleasant color of green. It has to do with the proteins within the egg that aggregate and coagulate together with the continuous application of heat.
The process of hard-boiling an egg involves denaturation. During this process, the amino acid chains found in the proteins in the egg are altered from their original, raw egg state …adding heat to these proteins initially causes the proteins to partially unfold or otherwise change their normal organized spatial structure. …if heat is applied long enough, you’ll get a greenish colorization around the yolk..