I tried the rainbow cake again today--this time using a 2 Pillsbury white cake mixes, flavored with vanilla and a bit of almond extract. Last time I used every bowl in my sister's kitchen, so this time I worked at streamlining things a bit by dividing the batter into the pans and then stirring in the color. Color didn't saturate, but stuck to the top level of the batter. Still a pretty cake but not up to my standards, so back to six separate bowls.
Loved the cake texture and flavor so much that I wouldn't consider baking the cake from scratch again. And, the cake came out of the pans much easier than the homemade cake. I used shortening to grease the pans, then a layer of parchment, and then a dusting of flour both times.
Still took 2-1/2 lbs. of powdered sugar to frost the cake! And, the weight of the frosted top layers caused the bottom three layers to compress to about 1/2 thickness.
I sent the rainbow cake home with the birthday girl, 'cuz I don't need the temptation.