You can add a shimmer by dusting with shimmer dust. Make a light grey by adding very small amounts of black paste color to white fondant, then dust with shimmer dust. For more metallic look, mix the dust into lemon extract or vodka; then paint on with a brush.
You can also purchase a metallic silver color that you can either airbrush on or apply with a brush. Any of the online cake decorating supply websites carry it. It is edible color and will give you the best silver, metallic look.