DAK, what it boils down to is you better be very familiar with a line of birds before attempting to cull them for footpad color at hatch. Poultry genetics is an extremely complicated subject and not everyone is going to agree on some things based on their own observations and experiences. Personally, I would not dream of culling for shank color or foot pad color at hatch. At six weeks, yes, maybe, but not before.
Others may feel differently, but if you are only hatching from a few birds why take a chance? It doesn\'t cost that much to raise them up to the age of six or so weeks. And eight weeks would be even better. If you run into problems with yellow skin epidermis the solution is to test mate all your breeders with yellow skinned birds. White is dominant, so if you get any chicks from the cross with yellow skin, you know the bird you tested is a carrier and is safe to cull.