I think yolk color depends on what the chicks have been eating
Late to the thread with my two penn'orth.
My student flatmate lived on, and was the son of an egg 'farmer' farm near Newbury.
He said they could make the yolk and shell ANY colour they liked with the correct chicken feed ingredients.
The only thing they couldn't do was to prevent 'odd shapes' and multiple yolks in young layers.
Hence when he returned from a visit to his home farm we (5 lads) had a gross of misshapen multi-yolked eggs.
Oh, and a barrel of his brother's home brew beer.
An enjoyable, sessionable, straight-forward simple clean ale.
Sometimes I think we forget these simple things in our strivings to make 'great beer'.
I'll get my coat....