My opinion: Mixing metals is ok when they are highlights, one on the other of a single item. eg Gold on silver ..to accentuate the design....

But for me, no way would I mix different metals on different accessories. It's a question of being stylish. So I have pewter with pewter: Silver with silver. I also would never use chrome with silver or pewter. Even though they are both 'white metals' the colour is so very different! I don't as yet have any brass or gilt but for the same reason I would not put together different 'yellow metals'

Likewise brown sporran with brown belt and shoes. Black with black etc.