The Room Rua class, of Tokanui School, asks :-
We have a hen's egg shell which is green. Why can an egg shell be green?
Neil Christensen, a poultry pathologist at Massey University, responded.
Shell colour, as you can see if you look at the shell side on, is given to shell by the chemicals laid down in the surface layers. This is done at the end of the time the shell spends in the shell gland (about 20 hours).
In hens, the shell colour is usually white (White Leghorns) or brown (Rhode Island Red) in which the colour is due to the pigment oophorin. In birds derived from the Awarana breed, the pigment is oocyan, which gives them a bluish tinge. If there is a lot of (ferrous) iron in the water, a greenish tinge can sometimes be given to the shell.
So in summary, the green pigment may be of genetic or nutritional origin.