Why do hard boiled egg yolks turn green when they are overcooked?

The green-gray color (and the whiff of sulfur smell that often accompanies it) comes from the reaction of iron in the egg yolk and sulfur in the egg white. When heated, the two can combine to make green-gray ferrous sulfide and hydrogen sulfide gas. To avoid getting a green yolk, cook your eggs just long enough to reach the desired doneness—no more. And quickly plunge the cooked eggs into cold water to stop the cooking process and minimize the iron-sulfur reaction.

It is a sulfuric reaction. It's not harmful though, and shouldn't change the taste at all. They're fine to eat.

Do you mean all green or just around the outside? If it is just the outside try breaking the shells as soon as you turn them off and run them under cold water. This stops the green rim. If they are all green then I don't know, maybe they are bad eggs.

I think it has something to do with over cooking the protein and it bruises. chef matty