This is the rainbow cake I made for my friend holly's 18th birthday. I used a mix of asda's and waitrose's liquid food colourings to dye each of the 6 layers of the basic sponge. The icing I used was a thick creamy frosting made from single cream, icing sugar and vanilla extract and I decorated it with smarties. It is very simple and homemade looking, but she loved it. Especially considering when she cut it she didnt know it was going to be so colourful.