Squeeze some red paint onto a plastic/paper plate, leaving room for the other colors. Add about 1/2 tsp of baking soda to the paint. Mix them together. The more baking soda, the fizzier the painting will be!
Using a paint brush, paint the first arc of the rainbow on a sheet of watercolor paper.
Finish mixing the rest of the colors on the plate, mixing in 1/2 tsp of baking soda to each one. Complete the rest of the rainbow by using a healthy amount of paint for each arch. The thicker the lines the better.
Fill a spray bottle with white vinegar and spray it onto the painting. The colors will fizz and start bleeding together to create a rainbow masterpiece.
Dry the painting on a wire rack. Your fizzy painting is complete!