According to horsetesting.com
Brown (and I'm not referring to Seal Brown) is a somewhat ambiguous term used to identify a number of horse coat colors but most often refers to a dark variation of Bay. We eventually will try to make changes to the calculator that will incorporate these variations. But for now, selecting Bay for the color brown will give you the most accurate calculations. However, colors such as liver chestnut or even smoky black are sometimes misidentified as brown. In which case Bay would obviously not be the correct selection.