Green egg yolk and brooding hen

I have had a few eggs that have green-tinged yolks when raw. Does anyone know why this could happen? They have only been 3-4 days old when cracked. I have 13 hens and 1 rooster. All of the other eggs have been beautiful with dark orange-yellow yolks. I collect my eggs every day and temperatures are in the teens. I have no idea which hen is laying these eggs. They eat mostly layer feed crumbles with the occasional snack of oatmeal, lettuce, cabbage, apples. Right now they are not free ranging as everything has been covered in a foot of snow for awhile now. All of the chickens seem to be in good health, but this is my first chicken experience so not sure what to look for, but they all act fine. I do have one brooding hen, which I do not know how to stop that. But like I said I am collecting the eggs every day. Any suggestions would be helpful. Thanks