I loved invisible ink when I was little. The idea of being able to pen secret messages only to be revealed later with a top-secret process fueled all sorts of imaginative play scenarios - all quite mysterious.
All you need to make invisible, magically revealed watercolor art is Clementine natural paint, watercolor brushes and paper, and a white crayon or candle.
Start by drawing a secret design on the white paper. Since it's white-on-white, it might even end up being a surprise to you!
Then, mix 1 T Clementine natural paint with 1/4 C water. Shake or stir well. Use watercolor, (soft bristle) brushes to paint over your hidden design.