Chestnut with darker mane and tail, brown grey hooves and a solid face. An uncommon variation is back right sock with grey hoof. Available as a single model #9010 from 1984-1988, in Bay, Chestnut and Black.
The Bitsy Breyer Jockey Set #1010 included Bitsy Breyer doll, Racing Tack, helmet and comb from 1984-1987, in Bay, Chestnut and Black.
In 1983, this model was released through Sears in the 'Breyer and the Little Bits' set #49C20130. The set included Bitsy Breyer with Western tack, a Buckskin Quarter Horse, Black Morgan, Chestnut Thoroughbred, and Bay Arabian.
In 1986, it was pictured in the 1986 JC Penney catalog, that the Chestnut, Black and Bay Thoroughbreds were available in a set #671-2483. However, the description mentions the set includes an Arabian Stallion, Palomino, and Bay Thoroughbred. Hopefully, I can find photos of what this set actually contained.