We once tried to turn a friend's sadly milky-white gravy brown for a Thanksgiving feast by mixing various food color drops, and only managed to get it to an equally unpalatable gray, so we are firm proponents of getting the right tool for the job. Fortunately, although black food color is hard to come by in most supermarkets, it is available in many well-stocked gourmet stores that specialize in cake decorating, as well as online.