I have only made fondant silver, and I do that by brushing on dry silver lustre dust or painting on silver lustre dust mixed with vodka. Not sure about buttercream?
There are no metallic food dyes. Metallics (gold, silver, bronze, copper) are only available in the luster dusts. They can be mixed with vodka, everclear, lemon extract, or clear vanilla extract to make a "paint" that can be applied to fondant, gumpaste, pastillage, and dried royal icing. They can also be brushed on dry to fondant, gumpaste, pastillage, and dried royal icing. If you are using a buttercream that crusts really well, you may be able to dry brush the luster dust onto the icing.
Also, here's a thread with info on silver airbrush paste http://www.cakecentral.com/cake-decorating-ftopict-46822.html
I haven't used the stuff, I just recalled having seen a discussion about it 
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