A chestnut brown horse’s coat color is influenced by a combination of the chestnut gene and the brown modifier. The chestnut gene produces a reddish-brown base coat, while the brown modifier darkens the coat to a rich, deep brown. Chestnut brown horses come in a range of shades, from light and golden to dark and chocolatey. They are known for their distinctive reddish-brown hues that can vary depending on lighting and grooming practices.