Line a 9×13-inch baking dish with parchment paper or grease lightly.

For the red layer: Melt 2 tablespoons of butter in a microwave-safe bowl. Add 5 cups of marshmallows and microwave for about 1 minute. Stir until smooth. Add red food coloring and mix well. Stir in 3 cups of Rice Krispies. Press the mixture evenly into the bottom of the prepared pan.

For the white layer: Repeat the same steps using 2 tablespoons butter, 5 cups marshmallows (add vanilla extract if using), and 3 cups Rice Krispies. Do not use any food coloring. Press this layer gently over the red layer.

For the blue layer: Repeat again using 2 tablespoons butter, 5 cups marshmallows, blue food coloring, and 3 cups Rice Krispies. Press evenly over the white layer.

Add sprinkles or M&Ms on top while the mixture is still slightly warm, if desired.

Let the treats cool completely at room temperature before cutting into squares.