I should start doing more of these.
You can use white gel food colouring to create really cool effects on dark fondant. My avatar all over the internet is this fondant Marvin. His head is made out of black fondant and his eyes are painted on with Wilton White-White:
You can also mix other colours with it. Here I mixed a tiny amount of green to make a spray paint colour to be painted on the brick wall (which was individually made and laid fondant bricks, glued in with royal icing):
If you mix neon colours – such as Americolor’s Electric line – into the white and then paint that on black fondant, you can get a really good neon sign effect:
All of these examples are from my 2010 Alien Film Festival show cake for the Austin cake show. On that page, you can also see how sometimes the white got muddied with other colours: that seems to happen if you try to paint the white over another painted colour, particularly black. So stick to painting it on fondant with black already mixed in, because that works better.
If you like this quick tip and want me to do more of them, let me know!