If I had a brown saddle I would definitely have a brown bridle, unless there is black in the saddle as well.
Regarding the saddle pad, if I were showing I would probably use white. Schooling, white, black or brown.
Oh, I just saw that you were wondering what color girth and not bridle. :) I guess for the girth it doesn't really matter but if you really wanted it to match then I would say brown. Although then it might be hard to find the same color brown as the saddle. :)