Dissolve one package of gelatin in 3/4 cup boiling water. Stir in 3/4 cup cold water. Spoon into pan and refrigerate until almost set, 45 minutes.
Dissolve another package of gelatin in 1/2 cup boiling water. Stir in 1/2 cup cold water and 1/2 cup evaporated milk. Spoon over first layer and refrigerate until almost set, 45 minutes.
Repeat steps 2 and 3 until all gelatin is used. Just before serving, top with whipped topping.