Q: Is a red legged Yorkshire Terrier a purebred?
While an introduction of another breed could certainly offset coloring, if you have an AKC registered Yorkie with proven lineage, then yes, he is a purebred.
Q: Are red legged Yorkies excluded from show?
While this super unique coat is a known and important factor with the breed, this is not an accepted color. The dog can be registered, of course. The red and black would be a major fault in show; so he would not be found in a show ring.
Q: How often should red legged Yorkies be added into a breeding program?
A: Sparingly. And many breeders do not implement this. Those that do, will perhaps do so once or twice in their breeding career. Enough breeders around the world know about this tactic and enough seem to avoid it, so that it is not done too often. If this were done too much, it would lead to an abundance of Yorkies that could/should not be bred and far too many that did not meet the standard.