I definitely think that the yolk color has to do with the fresh vegetable content in their diet. Our hens laid eggs that had bright orange yolks all summer and early fall when they had access to plenty of fresh grass and weeds as well as scraps from the vegetable garden and kitchen. However, now that there is much less vegetation growing, and there are rarely any fresh veggie scraps for them from the kitchen, their yolks are paler in color. This is the only change in their diet that I can think of that would affect the yolk color. Oh, that and less insects this time of year. But our hens get layer pellets, rolled seven grain blends, and scratch grains with cracked corn each day. So, I don't think the corn is what necessarily gives them the bright orange yolks alone.