Domestic dogs are a bit of an odd creature for sure.
They’re certainly the grey wolf’s closest relative, to the point that many studies classify them as the same species, just as different subspecies. In fact, domestic dogs and Holarctic grey wolves are more closely related to each other than either are to Indian and Himalayan wolves.
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“The data suggest that an ancient, now extinct, central European population of wolves was directly ancestral to domestic dogs.” - https://science.sciencemag.org/content/342/6160/871
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“The domestic dog (Canis lupus familiaris or Canis familiaris) is a subspecies of the gray wolf (Canis lupus).” - https://www.uniprot.org/proteomes/UP000002254
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This reference for the dog’s genome data refers to it as C. lupus familiaris, aka a subspecies of the wolf - https://www.ncbi.nlm.nih.gov/projects/mapview/static/dogsearch.html
Many wolf populations in Eurasia have a long history of introgression from domestic dogs. Black wolves in North America have black coats thanks to a domestic dog ancestor.
Pertaining to my original question above, should these ‘wolf’ populations be counted as valid wolves (bona-fide Canis lupus, morphologically and ecologically indistinguishable from other wolves), as domestic dogs, or as something of their own?
It’s possible that the ancestral population of wolves, from which domestic dogs descend, had golden fur (which some northern wolves sometimes have today) - here’s an interesting read on the topic. This would help explain why feral dogs tend to revert to a baseline of yellow-to-tan fur (as you mention with feral pigs and others that often return to wildtype-colouration).