Add 1/2 cup boiling water to 1 pkg. gelatin mix; stir 2 min. until completely dissolved.
Pour into 9-inch square pan sprayed with cooking spray; top with grapes. Refrigerate 20 min. or until gelatin is set but not firm.
Meanwhile, add remaining boiling water to remaining gelatin mixes; stir 2 min. until completely dissolved.
Let stand at room temperature until ready to use.
Whisk sour cream into cooled gelatin in bowl until well blended; pour over gelatin in pan. Refrigerate 1 hour or until firm. Unmold before cutting to serve.
