IMO, just because they do not meet the "standard" as far as color, that does not mean they are not Yorkies.  What if you have a Yorkie that weighs 9 lbs?  Does that mean it is not a Yorkie because it does not meet the "standard?"  No, it just means you have a bigger Yorkie...same is true for Partis...they are still Yorkies, they just carry a variance of the gene that determines coat color.  Is it standard?  No, of course not, but some favor this look and breed 
for it.  
As far as the Biewers are concerned...I have learned not to accuse them of being Yorkies or Partis or anything other than a Biewer for fear of getting my head bitten off over something I really could care less about one way or the other...