You are doing the right thing, learning all you can before you jump in. IMO they are like a lot of other confusing features that some yorkies are born with. Whether is is rare and therefore expensive OR if it is a defect that one should avoid is all up to who you are talking to. If it is a Yorkie you are after and you want to play it safe you should take the advice of the YTCA. website is
www.ytca.org go to the FAQ section and read about off colored Yorkies.
another site is a long time breeder and Yorkie enthusiast. She was a very traditional and respected breeder. here is the link, scroll down for chocolate comments.
http://www.shooterdog.com/alexfaq3.htm
If I wanted a chocolate, or a parti, or a tri, or a Biewer, or a babydoll face or a tiny I would learn all that I could and proceed with caution. And if I felt fine with it, I would go for it.
Be sure this is what you want. Look at
adult pictures of them. Are you going to be able to handle it whenever someone asks you what kind of dog you have? It may not bother you but we have a lot of posters who have non standard Yorkies and get really offended when someone asks.
good luck