This was fun activity that I got from Roper Mountain Science Institute. They suggested red theater gel to reveal the message, which can get pricy. Another idea I heard was to ask movie theaters to donate left over 3D glasses, which also have a red filter. A guest speaker we had last year said red jello would work. Tried it this week and it worked like a charm. The students thought it was very cool.
I found my "attention challenged" students had trouble focusing on making their own secret messages so I was very glad that I had made extra copies of the sheets from the GEMS Color Analyzer book that they could color and then check under the jello.