Technically, a green ring around your boiled egg yolk won’t harm you. The green or grayish ring is the result of a chemical reaction between iron in the yolk and sulfur in the egg white. But if green isn’t your color, you can avoid this unsightly phenomenon by not overcooking the egg and cooling it down quickly when you remove it from the stove.
No More Green Eggs and Ham
To avoid green yolks, simmer -- do not boil -- eggs in water at a temperature between 185 and 190 degrees Fahrenheit. Once the water comes to a steady simmer, turn the heat off, cover the pot and let the eggs sit. Soft-boiled eggs take 4 to 5 minutes and hard-boiled eggs should sit for 14 to 17 minutes. Once they are done, immediately run cold running water over them or submerge them in ice water.