I think it all depends on what region of the world you are in and what discipline or group you are most associated with. Many of the old cowboy types will call everything sorrel. However, many english riders call everything chesnut. I personally consider the lighter ones sorrel and the darker ones chesnut. I would call your mare chesnut because her color is a little deeper red than many. I consider the sorrel to be the more orangey color but you will get a different answer every time you ask a different person.