Why are facing difficulty in peeling hard-boiled eggs sometimes?

Sometimes it is easy peeling hard-boiled eggs , and at other times we face difficulty in the peel , why? It also turns the color of the yolk in hard-boiled eggs in some times to Green Is eating safe ? And what the reason for turning the yolk to the green ?

When fresh eggs , peel after boiling be very difficult so it does not boil eggs preferably be fresh when the eggs are old peel easily and so it is very suitable for the process of boiling .
Why the fresh eggs difficult peeling after boiling is that the room air ( and in the cavity is broad at the bottom of the egg ) be the size of a small , but in the case of the old Alad be the size of a large room air and thus be peeled easy.
In some times be a hard-boiled egg yolk in a tilted green color and is the result of the interaction of sulfur and iron found within the eggs because of the increased duration of boiling eggs and subjected to heat and there is no harm from eating the yolk when it is green.