For at home riding or just lessons I think it is fine (personally I like having my tack matching always), but for showing it is a must that the leather products you have are either all black or all brown.
It's not that big of a deal. People mix and match tack colours all the time! I have a black bridle and a brown saddle. If you're really worried about it then you can buy a black bridle for showing. It honestly doesn't matter it's just your personal opinion whether or not you want it to match.