Nutrena Naturewise adds marigold flower extract to their feed, which helps darken yolks. I bought a bunch of pumpkins last week cause they were on sale at aldis. Gave the hens several pumpkins already, & noticed a darker richer looking yolk almost immediately (my hens are free range). Tis the season for pumpkins, so u might try that too, esp since as a bonus pumpkins are very nutritious.