It can also be influenced by what you feed them. A friend of mine grew up with chickens. Her mother sold the eggs for a decent return. One day, when she was a kid, but old enough to know better, she and her partner in chaos decided to make the chicken lay eggs already colored for Easter. They fed the chickens copious amounts of blue dye. Yes, it worked. For weeks, the eggs came out sky blue. At first, the yolks were tinged slightly green. The blue egg shells took weeks to completely fade back to white. 😆