In a large pot over medium heat, melt the butter. Add the marshmallows, stirring constantly until melted. Stir in the food dye. Remove from heat and pour in the cereal, stirring to combine. Spray a sheet of wax paper with non-stick cooking spray, pour the mixture on the paper and flatten with a lightly sprayed spoon; let cool for 5-10 minutes. Once cooled, you can begin forming your brains or using a cookie cutter, cut out your pumpkins. Optionally, you can use green food dye for the stem of the pumpkin.