Hi there,
One of the things you could ask "this" breeder is to show you some pics of the previous litter as adults. Also not too sure if this is a repeat breeding ie - same dam and sire, or a different sire. And often times the Yorkies adult coat can lighten up due to a)breeding and or b) just ageing/over time.
If you look at show pictures you can better familiarize yourself with some of the "different" looks in the different breed lines.
www.ytca.org has a breeder listing by State.