Prep the Pan and Ingredients: Spray a 9x9-inch baking pan with nonstick cooking spray. Divide the Rice Krispies cereal into three portions of 2 cups each. Divide the marshmallows into three equal portions, about 5 ⅓ ounces each.
Nonstick cooking spray, 6 cups Rice Krispies cereal, 16 ounces marshmallows
Make the Blue Layer: Add one portion of marshmallows and 1 tablespoon butter to a microwave-safe bowl. Microwave in 20- to 30-second bursts, stirring between each one, until the marshmallows are melted and smooth. Stir in blue gel food coloring until the color looks bright and even.
3 tablespoons butter
Add the Blue Cereal Layer: Add 2 cups Rice Krispies cereal to the blue marshmallow mixture and stir until the cereal is evenly coated. Press the mixture into the bottom of the prepared pan. Use a piece of wax paper or lightly greased hands so you do not become permanently attached to the dessert.
Blue gel food coloring
Make the White Layer: Add the second portion of marshmallows and 1 tablespoon butter to a clean microwave-safe bowl. Microwave in 20- to 30-second bursts, stirring until melted and smooth. Do not add food coloring to this layer.
Add the White Cereal Layer: Stir 2 cups Rice Krispies cereal into the plain marshmallow mixture until evenly coated. Gently press the white layer over the blue layer, spreading it evenly without packing it down too hard.
Make the Red Layer: Add the final portion of marshmallows and the last 1 tablespoon butter to a clean microwave-safe bowl. Microwave in 20- to 30-second bursts, stirring until melted and smooth. Stir in red gel food coloring until the color is bright and evenly mixed.
Red gel food coloring
Add the Red Cereal Layer: Stir in the final 2 cups Rice Krispies cereal until fully coated. Press the red mixture evenly over the white layer to finish the red, white, and blue stack.
Cool and Slice: Let the treats cool completely at room temperature until firm. Cut into 12 squares and serve. Store leftovers in an airtight container at room temperature for 2 to 3 days.