So, after the 'success' of my first rainbow cake, my best friends niece chose to have one for her birthday, too, and as it was a joint party- the calling was for two!
Although I would never make 2-6-level cakes in one day again, the girls were over the moon which made up for my rather stressful day! And the cakes were infinitely better than my first attempt!
The thing I struggled with is the colouring: growing up, we had those liquid food colours that would dye anything any colour with just a few drops (think cakes, icing, clothes, fingers...) however, now- probably because of all this e number fuss- these have been discontinued and replaced with gel food colouring- which takes about half a tube to colour one cake, and begins to change the flavour!
So what I have found are these colour pastes from the wonderful Cake Craft World. They're a little more expensive, but you only need the tiniest amount so they last for ages- plus, they give a really nice rich colour rather than the slightly fake colours from the dyes.
Each layer of cake is just less than half of a victoria sponge mix, but I trimmed all of the edges off after baking to help them to sit flatter. I also use baking butter for the cakes, but real butter for the buttercream. I used 3 of each colour sugar paste to cover the cakes, which left me with plenty for decorations and a bit left over.