This is from a difference in food. If you don’t give your chickens grass and other natural foods and just give them grain their yolks will be pale yellow instead of the deep orange color.
It's a variety of things really, but the biggest one is diet. There are a bunch of chicken feeds that include a source high in carotenoids in order to change the color of the yolk.
My father-in-law has chickens that lay a rainbow assortment of egg colors, and their yolks are all deep orange. You’re totally right—it’s diet! They are fed chimken feed, fruits, and other assorted safe-for-chicken treats. I love them.