Cut 1/2-inch-thick slice from rounded bottom of watermelon half to prevent it from rolling around. Scoop watermelon pulp into large bowl. Reserve shell. Blend pulp, in batches, in blender until smooth. Pour each batch of juice into separate large bowl.
Pour 2 cups juice into saucepan. Bring to boil. Remove from heat. Add dry gelatin mixes. Stir 2 min until completely dissolved. Add to remaining juice in large bowl. Mix well.
Refrigerate 45 min or until thickened, stirring every 15 min. Pour into watermelon shell. Refrigerate 3 hours or until firm. When gelatin is firm, cut away any watermelon rind that extends above the gelatin layer. Serves 16