Yolk color is a function of xanthophyll in the diet. Backyard chickens tend to get food scraps that contain more xanthophyll than there is in commercial feed, which is why backyard eggs yolks tend to be more orange. If you want more orange in your yolks, you can feed your chickens stuff like marigolds or cooked carrots.
We'd feed birds crab shells every second day for a week or two. When the hard boiled eggs were cut open we'd have concentric red/orange rings in the yolk.
