According to "Comparative Coat Color Genetics" by Castle, there
actually is a difference between Chestnut and Sorrel. A sorrel is a
non-agouti, or self-color. The hair is the same color from root to tip.
Chestnut is an agouti color. The root and shaft are red, but there is a
yellow band just below the tip. Horse breed associations often do not
acknowledge these differences.