If this is the effect you're going for on fondant, it is achieved by mixing pearl dust with vodka and painting it on with a brush:
http://lamaisondesgateaux.blogspot.com/2011/07/bonne-fete-mili.html
http://lamaisondesgateaux.blogspot.com/2011/06/bonne-fete-crystal.html
I don't know how you'd achieve the same effect on buttercream.
On the jewelery box I used Wilton's white pearl dust over white fondant and for the gold highlights I used CK old gold over white, on the make-up bag I used Wilton's pink pearl dust over pink fondant (the same pink as the lipstick tube) and for the gold trim I used CK old gold over white. In both cases I mixed the powder color with vodka to make a fairly thick paint. I use the CK product for the gold because I find the Wilton gold to look weak and washed out.
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