Settle an Argument:What is "Sorrel?"

Half of the people in our barn say that sorrel is a red chestnut (that's what I thought when I was a kid and I had a red chestnut horse), and half say that sorrel is a lightish chestnut with a lighter mane and tail and lighter hair around the edges, legs, muzzle, etc., like you find in some draft horses. Settle the argument...who is right?