How long to boil eggs for firm yolk?
How long to boil egg for green yolk?
Place eggs in a single layer in saucepan. 
Cover pan and bring to boil; turn off the burner.
Let stand for 15 minutes for large eggs (12 minutes for medium eggs and 18 minutes for extra large eggs).
Drain. Immediately run cold water over the eggs.
This helps the green from forming around the yolks.
Store hard cooked eggs in the refrigerator and use within 1 week.