We had quite the friendly debate going on whether the boots which aren’t black have to match the coat. This was USEF’s response.
Brown boots are allowed in competition, provided that they are in line with the international HSV scale, and a conservative color jacket (let’s say navy in this case) can be worn with them in classes Fourth level and below. However, if competing above Fourth level, you must wear black riding boots unless the boots are the same color as the coat (and in line with the HSV scale); please note that this is also the same for FEI CDI events and FEI Dressage Rules as well.