Maybe this only adds to the confusion or maybe it helps to put things in context. When it comes to the partis and their origins, one must consider that there were crosses being made early on and that dogs from those crosses most likely made it into the KC stud books as Yorkshire Terriers.
Though many breeders certainly knew what they were looking for in their breeds early on and many breeds are much older than the dog show era, there were no 'pedigrees' as we know them today, nor official breed standards until they were drawn up by breed clubs as they were formed in the late 1800's. The 'broken haired scotch terrier' class encompassed many different dogs, including the Yorkshire and the Maltese. This mingling of different dogs into one class would have been a ripe situation for cross-breeding dogs.The first organized dog show was held in 1859. The Kennel Club had its founding in 1873. The first volume of the stud book covered 1859-1873. The first Yorkshire Terrier was entered in the stud book in 1874 and the first YT breed club started in 1898. Huddersfield Ben was born in 1865 and died in 1871. This is the backdrop for for our dogs becoming what they are today.
As things became more organized, it's easy to see how some of the crosses could have made it into the stud books and their influence may carry on today. There, however, has never been a written standard for the Yorkshire Terrier that included any color other than blue and tan.
Another similar example is with the Maltese. At one time, there were colors other than white. In the early 1900's, Maltese were crossed with poodles in America and both breeds are still affected by traits from the other. I find this 'interesting' and not completely unrelated to the parti discussion.
Here are some links.......
Yorkshire Terriers History of the Kennel Club - The Kennel Club The Maltese Breed History