Sounds like parti yorkies. Both mom and dad would both have to carry the parti gene in order for them to have parti colored pups. Some of the black and tan pups may be parti gene carriers too. Do any of the traditional colored pups have white chests, chins, throats or toes?
Attached is a photo of my parti pup, her sire is a full parti and mother is a parti carrier.
You can read more about the parti gene on my website (my information needs to be updated some) but maybe it will help. Please feel free to PM me if you have any questions, I'd love to learn more about your parti yorkies.
http://members.aol.com/pinehaven/miniyork.html
Please share photos when you get a chance ... Congratulations!