Hello and welcome to YT - I have no idea on the demand for chocolate Yorkies, but I do know there is a member here with a chocolate and he is adorable. Since you have never heard of them before and you breed, I would guess that they are not sought after or you would have had people contacting you and inquiring about them. Just my opinion, I don't know for certain. I'm sure others will offer their opinions. Again, welcome to YT.
