The green colouration (which usually occurs as a thin layer in the yolk where it meets the white of an egg) is caused by the reaction of air with the sulphur in the yolk.
The solution:
Once you've boiled your egg as long as you want empty the hot water down the sink and immediately fill the pan with cold water. Crack the egg underwater and the air doesn't get into the egg and you get a perfect boiled egg with a lovely clear white and a pure yellow yolk (I believe the sulphur ends up in the water).
When Bobbins makes boiled eggs she puts the eggs in a pan of cold water and turns the fire on under them. As soon as the water in the pan starts to boil, turn the fire off and cover the pan. Let it sit on the stove until the water is cold. The eggs will be boiled and the yolk will be yellow.
