Welcome to cake decorating and to the forum! Where do you live?
Getting to your question --
If you are speaking of food colors that give a metallic finish to your decorations, there are none. What is used is either called Luster Dust or metal dust, depending on what is allowed to be sold in your country.
Luster dust is an edible compound that offers a metallic finish. Metal dust is actual food-grade metal powders that are applied to the food product.
On the other hand, if you are speaking of food colors that color buttercream icing to tones of gold or bronze, you can purchase gold and copper food color from this site; for bronze you would have to mix your colors until you reach a desired shade.
Silver buttercream has been discussed on this forum a few times over the past months. Since you are new, let me tell you what the consensus is:
Attempts have been made at making silver icing, and according to what I have read, all attempts have failed. They turn up shades of gray, green, purple, or even blue.
If you some day figure out how to make silver buttercream by just using food colors, please let us know. You will become quite popular!
Good luck with your decorating!
Theresa 
Just wanted to chime in about the luster dust. The FDA here in the states considers it non-food approved meaning they don't think they should be ingested and you are supposed to tell your clients that as well. They are non-toxic and I don't believe the same regulations/recommendations are in place in Europe. And I don't think anyone is going to die from eating anything that is dusted with these powders but that's just me and what the heck do I know!
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