Chestnut (ee) is one of the two foundation colors, the other being black. Ponies are either black or chestnut with other genes modifying the base colors. Chestnut is a recessive gene so a pony must have two copies of chestnut in order to be chestnut and no copy of the black gene. Two chestnut ponies cannot produce anything but chestnut. A chestnut and a black can produce a bay.
There are many shades of chestnut from very light to very dark. Extremely dark sooty chestnuts are sometimes called "black chestnut". Chestnut or shades of it is called sorrel by some.
Flaxen modifies chestnut's manes and tails to make them a cream or white color. The genetics of flaxen is not yet known, however one study in Morgan Horses indicates it is inherited.
Typical shade of chestnut. Phantom Mist, Picture by Amanda Geci.
Red chestnut. Ginger of Assateague, Picture courtesy of the National Chincoteague Pony Association.
Chestnut minimal sabino as a foal and as an adult. Kayak, Foal picture by Amanda Geci, Adult picture courtesy of Barbara Steele.
Chestnut tobiano with flaxen mane and tail. Cezanne, Picture by Amanda Geci.
Light chestnut tobiano. Butterfly Kisses, Picture by Amanda Geci.
Dark chestnut. His dam and sister were also dark chestnuts. Rainy's Boy, Picture by Amanda Geci.
Dark chestnut. Wild mare, Picture by Amanda Geci.
Chestnut foal coat. Wild foal, Picture by Amanda Geci.
Flaxen chestnut. Wild mare, Picture by Amanda Geci.
Flaxen chestnut. Historical reference from 1967. Wild pony, Screen shot from a Wild Kingdom episode.