Place cake layers, top side up, in two 9" round cake pans.
Pierce cake with skewers at 1/2" intervals.
Stir 1/2 cup of the boiling water into each flavor gelatin in separate bowls, stir for 2 minutes or until completely dissolved.
Start pouring each different color gelatin into the various holes in both cake layers. Only pour one color into each hole. Keep pouring until all holes are filled with color. Refrigerate 3 hours.
Dip 1 cake pan in warm water 10 seconds; unmold onto serving plate.
Spread with about 1 cup of the whipped topping.
Unmold second cake layer; place on first cake layer.
Frost with remaining whipped topping.
Refrigerate until whipped topping is firm.
Place egg white in a medium mixing bowl and beat with an electric mixer to form soft peaks.
Add icing sugar and lemon juice and stir with a spatula to combine.
You want your mixture to be a thick but still running paste, if it is too thin it will run right off the cake and if it is too thick it won't mix and drip down the sides.
Split mixture into 5 bowls and add color to each bowl (orange, red, purple, blue and green).
Carefully drizzle small amounts of each color over the top of the cake. Start from the center and work your way out, this makes it easier to gauge when there is enough icing for it to just start over-flowing around the edge of the cake.