Victoria just asked me this while we were dyeing eggs on Saturday. Now I know!
From the Anne Marie’s Chemistry blog on about.com…..
Why Egg Yolks Turn Green
Thursday April 9, 2009
If you’re dyeing Easter eggs this weekend, or just boiling eggs to eat, you may want to avoid getting the green ring around the egg yolk. The green ring forms when you overheat the egg, causing hydrogen and sulfur in the egg white to react and form hydrogen sulfide gas. The hydrogen sulfide reacts with iron in the egg yolk to form a grayish-green compound where the white and yolk meet. While the color isn’t particularly appetizing, it’s fine to eat. You can keep the yolk from turning green by chilling the eggs as soon as they have finished cooking. One way to do this is by running cold water over the hot eggs as soon as the cooking time has elapsed.