Do you mean dogs like Thor?
Some are a mix, but some pure yorkies turn out that way too. It's just an uncommon coloring. I saw a pure bred yorkie the other day that was completely tan, no black at all. If you do a search on "platinum" on YT (search box is at the bottom of the page), some threads on "platinum" yorkies should pop up. I remember in particular one picture of a little platinum breeding female. She was just gorgeous.