The same rules of dress apply to the kilt as apply to ordinary dress: brown shoes, brown belt; black shoes, black belt. Ditto your sporran. As far as buckles go, well, highly polished brass will look better with brown leather. And if you really want to get down to the finer points, your watch should be of the same metal as your ring, and a gentleman should never wear more than two rings: a signet ring and, if married, a wedding band.