If you have a reddish-brown horse, you can call it a sorrel. If your favorite riding boots are the same color as your horse, you can describe them as sorrel too.
The word sorrel is a homograph with two distinct meanings and wordways into English. You can go for either the reddish brown color (or a horse that's got it) on the one hand, or any of various plants with sour juice. Both words came via French, and are ultimately from different Germanic roots. The color sorrel comes from sor, "yellowish-brown" in Middle French, while the horse of the same name might more commonly be described as "chestnut."