I have a cake order coming up for a punk rock theme. The party is going to have blacklights and everything that will glow in the dark. I thought it would be so cool to have painting on the cake that glows also. I have heard of using tonic water but not sure how to use it and if it would work on a fondant cake. Seems like it would make the fondant sticky.

that sounds so cool. Can't wait to see answers. Thaks for posting this topic.

tonic water is mixed with piping gel to paint or pipe on.

I got this recipe here on CC, but it didn't give measurements.

Put clear piping gel in a saucepan and heat until smooth.
Add tonic water.
Add plain gelatin.
Heat until thick.
Let stand for 2 hours.

I assume you can add whatever color you want to the tonic water. HTH!

Following! How much tonic water and gelatin do you add to piping gel?