@MysteryChicken is far more  knowledgeable about genetics than I am. But as I understand it birds have only 2 leg color: black and white. But then a second color gets added, sort of overtop of the base color. Therefore a chicken with white legs (shanks) will have white soles, even if a dark color had been “added” and they appear to have dark legs. A bird with true black legs will have black soles.
You’re welcome. I’m glad you asked your question because I am learning new things, too. I find chicken genetics to be both fascinating as well as overwhelmingly mind-boggling 

.