I would start by asking the breeder if he typically has yellow legs right from the start or if he feeds a specific finishing feed to enhance them. I'm not into Wyandottes, so I don't know the ropes there for what to expect.
In reading Plymouth Rock discussions, some mention that a high enough level of corn will make the White Rocks a little brassy if they don't have the special white gene, so it could be possible that too much corn might detract from the white on your Wyandotte, I honestly don't know. Another question for the breeder I'd say.
What you can do is give her everything possible feed-wise that may help, and if it doesn't, she's just faulty in leg color and that's that. I don't think it's uncommon, many struggle with getting consistent yellow legs in lines. I would probably feed a really quality high protein feed like gamebird (you want animal protein in the ingredients) and add some marigold and soaked alfalfa pellets and enough scratch grain to bring the overall protein level down around 20%. That should give you enough carotenoids to punch up whatever color she may have.