I had to do two. You know this, right?
I decided to go a little more subtle for this one:
I painted each nail black (Pure Ice "Touch me Here"), then sponged on a combination of black, purple (Wet n Wild "On a Trip"), silver (Wet n Wild "Metalica", and white (Maybelline Color Show "Porcelain Party").
For the Bat signal, I used Sally Hansen Xtreme Wear "Night Lights" with a couple drops of "Porcelain Party" added for opacity, and drew it on with a toothpick. Once it was dry, I did add a thin layer of glow in the dark acrylic paint, just where the grey is (note to self: top coat keeps glow in the dark paint from charging/glowing).