Melt 3 tablespoons butter in a large saucepan over low heat.
Add the 10-ounce package of marshmallows and stir continuously until completely melted and smooth. You can also melt the marshmallows and butter together in the microwave in 30-second intervals, stirring after each interval.
Remove the saucepan from the heat. Stir in 4 drops yellow food coloring and 2 drops red food coloring until the marshmallow mixture is evenly colored orange.
Gradually add 6 cups Rice Krispie cereal, stirring gently until all of the cereal is evenly coated with the marshmallow mixture.
Spread the mixture evenly onto a large jelly roll pan or baking sheet. Allow it to cool at room temperature for at least 30 minutes. Do not refrigerate, as chilling can make the cereal mixture too hard.
Once cool enough to handle, divide the mixture into 24 portions and shape each portion into a ball approximately 2 inches in diameter.
Cut 12 Tootsie Rolls in half to make 24 pieces. Press one piece into the center of each Rice Krispie ball to create the pumpkin stem.
Cut 2 green Fruit Roll-Ups into small squares, then cut the squares diagonally to create small triangles.
Place one or two green Fruit Roll-Up triangles next to each Tootsie Roll stem to create the pumpkin leaves.
Allow the pumpkin Rice Krispie treats to cool completely before serving.