Basically the black shanks are dominant.
......there are two layers of skin on the shanks,the dermis (inner layer) & the epidermis (outer layer). Various combination of genes give a variety of colours. The main genes involved are the gene at the e-locus, presence or lack of the white skin gene & a gene called Id (which stands for inhibitor of dermal melanin).
A bird which is extended black (E) at the e-locus, does not carry the (sex linked) Id gene but does carry the white skin gene (W+) will have black shanks with white soles of feet. If it does not have the white skin gene its feet will have yellow soles.
A bird which has the more recessive alleles at the e-locus, does not carry Id, & does not carry the white skin gene will have willow coloured legs.