Since it is not recogized by AKC or - YTCA ...
http://ytca.org/faq.html - and even states that one should not breed one that is not a recognized color. Any reputable breeder would not promote an un true color.
JMO - I would never breed a choloclate yorkie, since they are not to standard - doesn't matter if it is dominent or recessive. It is still there and can pop back up any time.
T.