So when Lucy asked me to make a rainbow cake, I breathed a sigh of relief! The only trouble was I needed the neon food colouring, which you seem only to be able to get from the US. So I found some on ebay and the rest was easy.
I took inspiration from here and the rest we made up as we went along! We just made our favourite cake recipe:
25g baking powder
1 tspn vanilla essence