Mix the cake mix with the soda according to regular instructions on box. It will be lumpy afterward. Again, you can use any white cake recipe you want, this is just how I do it.
How To Rainbow:
Measure the total volume , then divide by 6 cups about 1 cup each.
Stir colour into each bowl with its own spoon. Drop in your first three colours, then work on the other pan with the last three colours. For Rainbow, 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 according to package directions. Let cool completely before moving to a wire rack.
For The Frosting:
Meanwhile, make your frosting. Mix the pudding mix in with the whipped topping for a few minutes. May dye with your favorite color.