I don't know if the claims made by the Biewer Terrier club would be such a good example of biewers not being purebred YT, due to the various scandals that have erupted within that club but the other Biewer clubs do feel that the Biewers are pure bred yorkshire terriers with the piebald gene.
The YT breed standard was made long before there was clear understanding on genetics and how recessive genes work. The parti, chocolate and gold colors have been seen in the breed since it's beginning and documented in early writings. Also noted in these early books from the late 1800 – mid 1900's indicates that it was common practice to dispose of and destroy incorrect coloring within litters. It happened in the old days and I've heard of this practice still going on even within the last 10 years.
The Yorkshire terrier breed began from dogs with no heritage and no recorded blood lines. The dogs used in the makeup of this breed carried unknown genes for other colors. The maltese has been noted since the beginning of the breed, as being one of the dogs thought to be used in the breeds makeup. The Maltese displays the maximum expression of the parti gene. Numerous, numerous early historical writings about the YT breed say this, including early AKC publications like my 1945 “The Complete Book of Dog” which is a book of the different AKC breeds published for AKC and the content of each breed has been drawn up by the breeds club. I'm not sure what year the YT breed club changed their thoughts that the Maltese was used in the breeds make up but until at least the mid 1900's they felt the maltese was one of the dogs used. See attached.
The skye also thought to have been used in the makeup, come in “fawn” coloring which is a golden coat with some black hairs intermingled in the coat. This can be seen on this Skye terrier breeders webiste
portrees It shows some black puppies and some that are dark gold with black hairs most noticeable seen along the topline. Many of the Golden colored yorkies are actually “fawn” or sable in color. Please see photos of 2 different fawn colored Yorkies owned by me.
The waterside terrier is now thought by the YT club to be one of the 3 breeds used in the make up of the YT breed. The Waterside is by the Otterhound and in addition to black and tan coloring, the Otterhounds come in parti, chocolate and golden!
There are Yorkshire terriers that are born traditional colored black and tan who “run gold” as an adult as Lorraine says but there are also Fawn colored yorkies born and clear coated gold and blondes born. As you can see from the photos I've posted they are born golden and stay golden.
Standards in any breed, canine, equine, feline ... are a wonderful guide for achieving what's thought to be the ideal look for the show ring, of course standards can and have changed over the years. My feelings (and many others feelings) are that this is a breed registry and if an off color shows up in a litter of traditional colored dogs, they should have as much rights as their traditional/standard colored siblings have.