Coat color DNA testing in dogs: Theory meets practice
This article is behind a pay firewall, but it describes commercial DNA testing for coat color genes in dogs and has a table of alleles that have been tested for in various breeds, including the Yorkshire Terrier. Yorkies have been tested for the presence of "sp" (Parti color), "b" (brown), and "e" ("Gold Dust") alleles. The only problem is that these alleles have very specific DNA fingerprints, and they arose in other breeds. They could not have arisen spontaneously in the Yorkshire Terrier.