Place cake layers, top sides up, in 2 clean layer pans.
Prick each cake with utility fork at 1/2 inch intervals.
Combine 1 cup boiling water per box of jello. Carefully spoon jello mixture over each cake. Repeat until all colors are used.
Chill 3 to 4 hours.
Dip one cake pan in warm water for 10 seconds and turn out onto serving plate.
Top with one cup of the whipped topping. Unmold second cake layer and place carefully on 1st layer. Frost top and sides with remaining whipped topping.
Chill. Garnish as desired.