My friend was in a quandry a few days ago because he handbag had silver accents and she was wearing a gold necklace. The fashion gods were going to strike her down!
I assured her that mixing metals was more than allowed; its encouraged. Colors in metals compliment one another quite nicely whether piled on top of one another or in the same piece.
This article in Getting Jessed Up gives some really great examples of daring metal mixes that obviously work.
Some examples of David Young jewelry: