Why do cooked egg yolks sometimes have a green ring around them?

This is a natural condition and not at all dangerous. Yolks naturally contain some small amounts of sulpfur (which is released when they are cooked - or when eggs rot, hence the particular smell of "rotting eggs". This sulfur combines with hydrogen present in the egg whites and that produces the green ring. It can also be causes by a high amount or iron in the water (again, not dangerous).

A green-blue color is usually caused by simply cooking the eggs too long.

How to cook hard boiled eggs to avoid the green rings

Many state university extension services recommend the following cooking method for hard boiled eggs to avoid the green rings: The following comes from the University of Wisconsin:

1. Place eggs in a single layer in a saucepan.
2.  Add cold tap water to cover by at least 1 inch above the eggs.
3. Place the pan on the stove and bring to a boil.
4. Turn off the burner. (You can leave the pan on the burner, just make sure it's turned completely off.)
5. Let stand for 12 minutes for large eggs (11 minutes for medium eggs and 15 minutes for extra large eggs).
6. Immediately plunge eggs in ice water to stop the cooking process.
7. Store hard cooked eggs in the refrigerator and use within 1 week.