Anybody know how to make silver buttercream? Or am I stuck having to make grey??!

You can buy silver or pearl lustre sprays - just spray it on once your cake is iced. I would do a test on both uncoloured and grey icing (just put some on some parchment or benchtop) to see what you like best.

Hmm. I guess I should have noted that I need the silver buttercream for piping a design on a cake. Want to make a silver tassel on a graduation cap....

If your butter cream is a good crusting one, you can do this: Color your butter cream grey, decorate, and when it is good and crusted paint back over it with silver luster dust that has been mixed with a clear alcohol based extract or with baker's alcohol or vodka.