That's great. It is just an anomally but that's good that they are letting you register them. Why shouldn't they, parents are, right?. I think that we haven't heard a lot of off colors in yorkies because breeders didn't want to let people know that they would have "weird" babies, so I'm sure that some just culled them out of the litter
Years ago, that's what they did with white Boxer pups, but now people are a little bit more flexible, and they give these babies the same love they give their "regular" yorkies...