Prepare a 9x13 pan by lining it with a sheet of parchment paper that overhangs on the sides. Spray the parchment with nonstick spray.
Place butter in a large microwave-safe bowl and microwave for 30 seconds to partially melt. Swirl the butter around the bowl to coat the sides, then add marshmallows. Microwave in 30-second increments, stirring each time until the marshmallows are melted and the mixture is smooth.
Stir in the vanilla and the food coloring until the color is uniform with no streaks. If you want a darker green, add more food coloring. Add the cereal and continue to stir until it is completely coated, with no dry cereal remaining.
Place the cereal into the 9x13 pan and use a large spoon that has been sprayed with nonstick spray to press the cereal and marshmallow mixture evenly into the pan. Top with sprinkles then refrigerate them for 30 minutes.
Remove from the refrigerator and lift the overhanging edges of the parchment to remove from treats from the pan and place on a cutting board.
Spray your cookie cutter with nonstick spray, then press firmly into the Rice Krispie bars. Remove the shape and transfer to a serving tray. Repeat until all of the bars are cut. Press the remaining scraps together to cut more trees, if desired.
Stovetop Instructions: Place the butter, marshmallows, and vanilla extract in a large pot. Heat over medium-low heat, stirring until the marshmallows are melted. Next mix in the food coloring, then stir in the cereal until it is completely coated.Storage: Store the bars refrigerated in an airtight container for 3-5 days.Food Coloring: The amount of food coloring needed for this recipe will depend on the type of coloring you use. A gel coloring is more concentrated, requiring fewer drops. While a generic liquid coloring needs more. Variations: Add mini M&Ms or crushed peppermint candies instead of the sprinkles. Shape into wreaths (with buttered hands) and press red hots into the treats for berries. Cut into squares or hearts and press a red heart candy in the center for Grinch treats. Dip in melted white or milk chocolate to coat the bottoms of the treats.