I mix my metals. My Texas A&M class ring is gold and my wedding band is 3 different shades of gold twisted together, and I'm not going to deny myself wearing silver (or leave my ring off when doing so). I also occasionally wear a titanium ring that my wife and I got for our 10th anniversary. I will mix any combination of these with a silver/nickel belt buckle or an antiqued bronze style buckle, and maybe even a brass sporran cantle. I don't think it's a big deal. Not for day wear, anyway.

In your case, for what I assume is a formal dinner, with gold badges, it's not like you'd have the option to switch over to a gold sporran cantle and kilt pin. As I've been told, those don't really exist in THCD. So mixing metals is pretty much a given. Each item is what it is. I wouldn't worry about it.