The AKC Breed Standard and YTCA Code of Ethics states that they do not recognize chocolate.
"This includes all gold, born blue, liver (also known as red or chocolate), and parti-colors. One of the reasons for avoiding breeding "off-colored" Yorkies is because it could be a genetic defect that may affect the dog's health. Some health problems can include, but aren't limited to, severe skin problems, allergies, total hair loss and in some cases long-term illness and/or death." See
Yorkshire Terrier Club of America (Awards)
Those that advertise this color as "RARE" are being dishonest in that they are just trying to enrich themselves with more money, in spite of the lack of pigment in the dogs they produce which could bring a genetic nightmare to the breed. These dogs should all be neutered. Shame on this person.