Place shaving cream in a large measuring cup or bowl.
Add food coloring or liquid watercolor paint and stir with a spoon.
Add cornstarch. Stir with a spoon for as long as you can, then knead the ingredients with your hands until well combined.
Try pressing the dough into a ball to test its consistency. If the dough is too dry, add more shaving cream; if it's too wet, add more cornstarch.
Start playing immediately. Store in a sealed sandwich bag when not in use.