In a small bowl, dissolve one package of gelatin in 3/4 cup boiling water. Add 3/4 cup cold water; stir. Spoon into a 13-in. x 9-in. dish coated with cooking spray. Chill until set but not firm, about 40 minutes.
In another bowl, dissolve another package of gelatin in 1/2 cup boiling water. Add 1/2 cup cold water and 1/2 cup milk; stir. Spoon over the first layer. Chill until set but not firm, about 40 minutes.
Repeat five times, alternating plain gelatin layers with creamy gelatin layers. Chill each layer until set but not firm before spooning next layer on top. Refrigerate entire salad overnight. Just before serving, spread top with whipped topping. Cut into squares to serve. Garnish with mint and fruit if desired. Yield: 15-20 servings.