Hope this helps a a bit.
A greenish ring around hard-cooked eggs in the result of sulphur and iron compounds in the egg reacting at the
surface of the yolk. Although the color may be a bit unappealing, the eggs are still wholesome and nutritious and
their flavor is unaffected. Greenish yolks can best be
avoided by using the proper cooking time and temperature and
by rapidly cooling the cooked eggs. Usually, eggs have been
cooked at too high a temperature, for too long a time, or
they have not been rapidly cooled following cooking.
Cooperative Extension Service
University of Illinois