In talking about shank color a person should consider two parts, the epidermis and the dermis. Some of the above affect only the epidermis. It is very possible to get good dark slate shanks on white birds. Slate is composed of a dark dermis and a more or less clear, transluscent epidermis. I would be more concerned with factors that dilute or prevent a dark dermis - like sex linked barring.
From Sellers' Website:
"Dominant white (symbol I) Incompletely dominant. Influences eye pigment. Inhibits black pigment, eumalanin. This gene is ‘leaky’ and will allow black specks through. Generally not as efficient at producing a solid white bird as are two copies of recessive white. Heterozygotes of dominant white, I/i+ are often grey with the grey color visible in the chick down. Dominant white dilutes, but does not eliminate, epidermal melanin."
Notice it does not say anything about diluting dermal melanin.
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