Combine a teaspoon of cornstarch with a Tablespoon of water in a medium mixing bowl. Stir until dissolved.
Add frosting to bowl and stir. Use a ratio of 1:3 frosting to cornstarch, to make your desired size batch.
Add the rest of the cornstarch a little at a time, stirring until it becomes difficult.
Finish kneading by hand.
Divide the dough into equal portions, one ball of dough for each color/flavor of Jello you're using. Add a Tablespoon of Jello powder to each ball of dough.
Knead until the Jello colors the balls of play dough.
Combine all the colors into one ball of rainbow play dough and have fun!