Personal preference is the best colour
As with KIIM I think black bridles look smart on any horse. However, I have a brown bridle that blends perfectly with my bay stb and it looks really nice.
My pick would still be black though unless you want one of those funky colours as a training bridle. Theyre cool :) if so then I would say blue or purple going by your girls colour :) maybe even yellow...or green would look good too...even red would contrast nicely...ah, I dunno. Im picturing every colour looking good at the moment lol