Using a large pot over medium to medium-high heat, add water and food coloring. Give it a good stir. Add the baking soda, give it a quick stir. Add the corn starch and give it a good stir until well combined. Continue to stir the Playdough continuously until it starts to take shape and pull away from the pan. Turn off heat. Using a spoon or spatula pull the dough out onto a clean surface. Let cool a few minutes and then knead until smooth. Store in a ziplock bag.