I did this cake with colored piping gel with a little bit of Wilton white coloring to make it opaque. But here's the kicker...I added a bit of Tonic water to thin out the gel. Under a black light, the colors glowed in the dark!
It was for a 13 year old who was celebrating her birthday at the bowling alley where they have Cosmic bowling. All the lights go out and only black lights are used. The cake was the talk of the bowling alley!