The whole mixing metals or mixing colors or layering of clothes with many extraneous pieces/accessories is a clever ploy...it means you probably need to buy more pieces of jewelry or clothing to carry it off...therefore you're spending more money. I mean you MIGHT already have all you need to make the "look" but more often than not you need just one more thing to bring it all together!
In terms of actually mixing metals and colors I tend to go with all cool or all warm...just don't cross that line.