Funny, i just saw a minute ago "white" purebred yorkie.
They say:
"Pappus to the most beautiful yorkie in Israel"
And than you can see her pic...(well, i belive she is the most beautiful yokrie for her owners, but imho..give some respect to the breed standard).
pic:
http://www.yad2.co.il/Pic/200607/12/...0712220736.jpg 
there is white, black, green, red yorkies
But this is not a standard.
i saw a 7 kg purebred yorkie ...(13 ebils), but again, it's not a standard.
So standard of course is possible...and those things are happened even with the best breeders, but those kind of dogs SHOULD cost much less money.
They are not better than other, and we will love them the same, but this is not a standard.