Microwave butter in large microwaveable bowl on HIGH 45 sec. or until melted.
Add marshmallows; toss to coat. Microwave 1-1/2 min. or until marshmallows are completely melted and mixture is well blended, stirring in food colorings after 45 sec.
Add cereal and candy corn; mix well. Press onto bottom of 13x9-inch pan sprayed with cooking spray.