Is it? I thought it is not known. Some people believe it's "behr" because of "behrlohga" - "bear den" but there is more plausible etymology for that being derived from actual Old Slavic "trash"/"dirt" than "behr + lohg".
But then it is derived from "brown" (dirt, garbage) in the same way as in English.
I doubt it has a close connection to "brown". "Behr-" in Slavic languages, when stands for any color at all, is, usually, for "white".