A common question when it comes to jewlrey is wether or not you can mix silver with gold. For a long time I would NEVER mix the two but now I am more open to the idea with a few exceptions...
Rule #1: Try to mix metals that are close to each other, like mixing gold and silver bangles or layering up silver and gold rings on one finger. Putting them this close will a) make it look like you are going for the metal clashing looking and b) blend the colours better.
Rule #2: Mixing gold shoes with a silver dress is ok if the accessories are done right but don't mix a silver top with a gold skirt. Since the clashing of colours is a big part of the outfit and so close together it wont look right, were are shoes and a skirt are further apart giving the eyes time to adjust.
Rule #3: If you are planning on mixing metals its a good idea to look for accessories that have a metal combination in them alrready, for example, a charm braclet with different metal charms. This will help each of the other peices of jewlery blend well with the outfit.