Why do some egg yolks turn green?

Generally the wierd green ring that forms on overcooked eggs reminds you of belly-button cheese is normal for things like, Egg McMuffins and your grandma’s deviled eggs.. But WHY man, WHY? Is it cool to eat?

According to AskMrBreakfast.com:

A greenish grey film will form on the surface of the yolk when the temperature of the yolk exceeds 158 degrees Fahrenheit. This discoloration is not unhealthful. It’s just a visual indicator of a natural chemical reaction… hydrogen sulphide from amino acids in the white of your egg (or albumen) is reacting with iron from the yolk which causes a film of ferrous sulphide to form on the yolk’s surface. If the film is thin, your egg will still taste fine.