Could you do three cake layers, with three layers of icing, all in different colours? I've seen that somwehere, maybe Pinterest... Anyway, it'd go

Red Icing
Orange Cake
Yellow Icing
Green Cake
Blue Icing
Purple Cake

Or, change all those colours to whatever the other layer is (so red icing becomes red cake, etc.) and have thick blue and white icing on top in a kind of sky-looking design so when you cut into it everyone goes "oooooh"

If you want a general cake tip - I don't know how often you make cakes - a good one is to always use room temperature ingredients (except probably milk, but even then you don't want it too cold) and a preheated oven.