Both of my boys are Parti mixes, but I wouldn't turn down a chance to own a Biewer, either!
The only really telling feature in Neo being a parti is the tiny little white soul patch on his chin, and if you look closely you'll see the white and dark speckled patch of skin on his chest. Connor looked like your standard yorkie baby, but by the time he came to me at 11 weeks he started sprouting white/silver hair on the top of his head. At 5-6 months after his first groom his entire body had turned silver! Only his muzzle is tan, now, and he is just gorgeous!
That being said- I resent anyone telling me that MY BOYS are NOT Yorkies! They may not be AKC or "ring quality", but they are most definately Yorkies! That is like telling an adopted person that they are not "really" family!
You know who you are.