2 boxes white cake mix
24 oz clear diet soda (sprite zero gingerale etc)
1 box gel food coloring
16 oz whipped topping
2 oz instant fat-free sugar-free pudding mix
Pour 2-16 oz cans of soda into the two boxes of cake mix. No eggs no oil no water no sweat. It will be lumpy.
Measure the batter evenly into 6 separate bowls. It does not have to be perfect.
Stir color into each bowl with its own spoon. For the first color into the pan measure out 2/3 to 3/4 of your mix (in this case about 1 c) as close to the middle as you can. Drop in your first three colors then work on the other pan with the last three colors. So if you are doing rainbow order the first pan should have red then orange then yellow and now the purple blue and green go into the second pan. As a recap this is so both layers are roughly the same size.
Bake the cake as instructed on the box.
Meanwhile make your frosting. Just mix the pudding mix in with the whipped topping for a few minutes. Dye if you want.
Frost your fat-free cake with your fat-free whipped frosting. Eat.