Reason for green layer on yolk in a boiled egg?

[Sarat5]

I still haven't figured out if the outer layer of the yolk in a boiled egg turns green because it's been over boiled or because its not fresh.

 

[keltillos]

The outer layer of the yolk turns green when the egg has been slightly over boiled. I have been told that has to do with a chemical reaction involving the sulfur in the yolk.

 

[neionix]

Don't worry. It's been overcooked. If you boil a bad egg, you won't be able to tell the difference till you taste it. It's also better to boil eggs closer to the expiration date because the membrane inside the egg has started to separate from the shell and makes it easier to peel.