I've been giving my girls carrot greens and fall/winter veggies like leftover kale, broccoli leaves. Anything I yank from the garden that's edible, I'll let them strip off the leaves first. My feed store also sells dried kelp meal which supposedly has the same effect... I haven't needed it so haven't tried it.
As others have noted the appeal of an orangey yolk is purely aesthetic. I love the look of it as well. I'm thinking for spring, of planting carrots all around the run perimeter, so the girls can nibble off some of the greens, though the local rabbits would probably put a stop to that real fast.
Mine had access to carrots this summer and would not touch em. Spinach and lettuce, that was a different story. Same with diakon radishes. They stripped those bare.