Nonstick cooking spray, for the baking dish
3 tablespoons unsalted butter, cut into tablespoons
One 10-ounce bag mini marshmallows
5 1/4 cups crispy rice cereal, such as Rice Krispies
1/2 teaspoon blue gel food coloring (see Cook's Note)
1/2 teaspoon red gel food coloring
1/4 cup mixed red, white and blue sprinkles
Gel food coloring delivers extra vibrant color but you can also use regular food coloring for these treats. You'll need more than the amounts called for here; keep adding drops until you achieve the desired color.