I'm doing a cake for a huge 30th b-day bash. She wants the bow to be blue and silver. Should I color the fondant gray first and then paint it with the lustre dust or just leave it white?

I wouldn't color the fondant gray. If you have time, get some confectioner's glaze. If you mix it with the alcholol and dust, it will give you a very nice shiny surface. If you can't get it, the dust and alcohol will be fine on it's own.