Preheat oven to 350.Prepare a 13" x 9" pan with cooking spray and flour; set aside. In a mixing bowl, combine cake mix, one package pudding mix, carbonated beverage, oil, eggs, and walnuts. Mix until moistened. Spread batter into prepared pan. Bake for 50 minutes, or until lightly browned on top. Cool. Meanwhile, to prepare frosting combine milk, remaining pudding mix, and green food coloring in a mixing bowl. Mix well. Fold in whipped topping. Spread over cooled cake. Refrigerate several hours before serving.