Melt 1 tablespoon butter in a microwave-safe bowl. You can also melt the butter in a saucepan over low heat.
Add 1/3 of the 10-ounce package of marshmallows to the melted butter. Microwave in 30-second intervals, stirring after each interval, until the marshmallows are completely melted and smooth.
Stir in the orange food coloring until the marshmallow mixture reaches the desired shade of orange.
Gradually add 2 cups Rice Krispie cereal, stirring until all the cereal is evenly coated.
Spread the mixture onto a large jelly roll pan or baking sheet and set aside to cool.
Repeat Steps 1–5 two more times, using the remaining 2/3 of the marshmallows, 2 tablespoons butter, and 4 cups Rice Krispie cereal. Color one batch green and the other purple.
Allow all three mixtures to cool at room temperature for at least 30 minutes. Do not refrigerate, as this can make the Rice Krispie mixture too hard.
Once the mixtures are cool enough to handle, divide them into 24 portions and shape each portion into a ball approximately 2 inches in diameter.
Gently press candy eyes into the sides of each Rice Krispie ball to create the monster faces.
Allow the monster Rice Krispie treats to cool completely before serving.