When I need silver, I will just use white icing & paint silver onto it once it's set... silver luster dust mixed with "confectionary laquer" - I forget what it's called... actually that's what I was told, and my supplier knew what I was talking about. I don't know if this is "the right way", but it works for me. Good luck!
I would also like to know how to make silver icing (specifically buttercream). I wonder if it's possible to mix silver luster dust in with the buttercream? Or is luster dust the kind that's not meant to be ingested?
Dear Dixiechick -
Welcome to the forum. I looked to see if you were new, because this subject comes up from time to time. I hope you enjoy this forum as much as the rest of us do.
Since you do not know, the past discussions revealed that you will never really achieve a silver icing. If you try to mix silver icing, it will look gray, or a very off shade of purple or blue.
As mentioned in the above posts, your best bet will be to use silver dust and paint it with Everklear alcohol (if available in your state) or clear lemon extract.
Theresa 
Definately luster dust. Also, I dont have an air compressor, but that might do it too.
I found this recipe for a silver and gold piping gel. I've never tried it so I have no idea if it will work or not, but if this method would work for what you need then try it out.
http://www.cakecentral.com/cake_recipe-2039-Piping-silver-and-gold.html
Also you can paint on Royal icing, fondant and gumpaste with the luster dust paste. I think it would just mostly goopy up your buttercream.
Here's the ratio I use for painting with dusts: 1 tsp. lemon extract (or vodka, everclear, etc.) to 1/4 tsp. dust. That will paint quite a bit (though I don't know exactly how much) of icing. I'd say that you should have plenty of dust in just one little container, but I don't know for sure.
I've never painted on buttercream, but my guess would be that if you wait until the buttercream has crusted, it should work just fine. Just try not to press too hard with the paintbrush. Please let us know how it works out.
I would think that the alcohol or the lemon you use to paint on lustre dust would react with the buttercream when it came in contact. I tried to "pick up" some confectioner's sugar from the surface of purple buttercream with a paint brush dipped in vodka and it actually changed the color of the icing in the places I touched (and it had been crusted for over an hour). Okay...I guess it is time to go to the "Test Kitchen" bakers...start your ovens!! 
msauer, you have a very good point. I didn't think about the fact that the extract/alcohol could react with the buttercream. Hmmmm. Hopefully someone here will know whether or not it works with buttercream. Thanks for bring that up!
Dixiechick -
You never mentioned whether you were using this to ice the entire cake or for ornamentation on the cake.
If you are using edible genuine metals, be aware that when mixed with water-based liquids, i.e. extract or low proof vodka, there is a chance that the metals will tarnish. What you see may not be a reaction to the buttercream, but to the water-based liquid used to paint it on with or the water content in the buttercream.
Theresa
I have painted on crusted buttercream with vodka and luster dust with no problems. The Vodka evaporates quite fast so it doesn't have time to hurt the buttercream and the color underneath shouldn't matter snce it is covered with the luster dust color. Here's a couple of picks of cakes where I painted gold or silver onto buttercream. You just press very lightly with your brush so as not to make an impression into the buttercream. I don't know if it would be practical to paint the entire cake though?
Wow ... those cakes are beautiful! I'm glad to know that it is possible to paint on buttercream. Thanks, Euphoriabakery!
Quote by @%username% on %date%
%body%