Happyhens, some hatcheries deliberately mix the NN stock for a "variety of colors"(McMurray catalog even said so, for many years). It appears to be mostly a color crossing thing in the hatchery stock as they largely look rather uniform in type but probably there was some outcrossing to other breeds*, causing a wide range of leg colors in the hatchery stock- white, yellow, slate, willow, black.
Leg color is due to three main factors- skin color on whole of body(white vs. yellow), presence or absence of a layer of pigments(independent of the body skin pigment) in the legs and finally, in some cases due to plumage color(the two types of black feathering genes also happen to put black pigment in the leg).
As to your bird in question- slate legs is just the combination of white body skin plus presence of a layer of pigmentation in the leg skin. Had the body skin been yellow, the legs would have been visually green/willow. It's the visual combination of the leg pigment + white skin or yellow skin.
Clear yellow or white legs are usually yellow skinned & white skinned respectively with a gene that says "no pigmentation in that layer of leg skin", so the body skin shows through on the legs without any extra color added.
*for example, white skin is dominant over yellow skin(which standardized NN are supposed to have), the presence of white and slate legs indicates there was an outcross to a white skinned breed/bird somewhere in the past(long time ago, I've been seeing both of these leg colors 20 years ago, probably went back even further.. saying this so nobody will think it had to be a very recent event). It also being dominant and not culled out really helped it to "float" down around for generations on end. Probably was not culled out because it didn't really matter and/or they wanted to preserve a wide range as part of variety of colors goal.
