Add confectioners sugar one cup at a time and beat on low until combined.
Generously spray six 9-inch round cake pans with cooking spray. One color at a time, pour the colored cake batters into the pan and bake in the preheated oven for 10 minutes per cake.
allow cakes to fully cool on wire rack after baking
[Start with purple and go up the rainbow back to red so you can stack the cakes on the platter as you go. I found that the cakes sometimes broke apart if I allowed them to sit on a rack or on wax paper while cooling.]