Line a 9-inch square pan with foil and spray with cooking spray. Spray a large bowl with cooking spray and add 6 cups of cereal to the bowl.

Spray a medium microwavable bowl with cooking spray. Add butter and marshmallows. Microwave uncovered on High for 1 minute 30 seconds to 2 minutes, stirring every 30 seconds, until smooth.

Pour the hot marshmallow mixture over the cereal and stir until the cereal is coated. Press firmly into the pan. Cool completely, about 30 minutes.

In a small bowl, stir the frosting and food colors to achieve the desired purple color. Spread the frosting evenly over the fully cooled cereal bars using an offset metal spatula.

Top with 1/3 cup marshmallows from the cereal and candy sprinkles.

Lift the bars from the pan and discard the foil. Cut into 4 rows by 4 rows. Store covered in an airtight container.