In a small bowl, stir ½ teaspoon borax into ¼ cup warm water until fully dissolved.
In another bowl, combine 1 tablespoon white glue, 1 tablespoon cornstarch, and 1 tablespoon warm water. Stir until smooth. Add a few drops of food coloring for your first color and mix well.
Split the glue mixture into six bowls. Add food coloring to each to make red, orange, yellow, green, blue, and purple. Mix until vibrant.
For each color, slowly add a little borax solution to the glue mixture while stirring. Keep stirring until it thickens into a rubbery dough.
Take a small piece of each color and stack them in rainbow order—red to purple. Gently press the layers together.
Roll the layered dough between your palms until smooth, firm, and round. Keep rolling for better bounce.
Place on parchment or a plate and let sit for 10–15 minutes to firm up before playing.