What color are their legs now? You can try foods high in beta carotene like pumpkins or carrots, probably cooked and IF it works, it'll will take a week or two.
Their legs are kind of fleshy colored. Seems I remember years ago when my sons raised them for 4-H we fed them corn meal with their food. What do you think about that?
Paprika from the grocery store is the most readily available and affordable source of concentrted carotein. Just mix it into the feed. I know it will turn yellow legs a brighter yellow but do not know if it will turn flesh color into yellow.
I dont think you can change the color of a chickens legs. Pretty sure they are genetically set. As raingarden said, you might be able to turn yellow legs brighter to a degree - just as you can make feathers shinier or egg yolks more orange, but I dont believe you can change the color of their legs short of dyeing them.