My vibrant rainbow cake is made with a lot of gel colour’s and I mean a lot. If you want the colour’s to be bright and vibrant. Just keep adding the gel colour’s until you get the desired brightness you are after as you can see I did the same thing to the vanilla butter cake sponge mix. To me this is how a rainbow cake colour’s should look, am I being bias no…….. a rainbow has very bright colour’s and does not look washed out in colour. Petal piping is done by using a piping bag, just simple pipe a reasonable size dot of the butter-cream and smear across it using the back of a spoon, making sure you overlap each smear. Being consistent with the dot amount is important other you will not get the same look of the petals and another thing measure your cake of the height so you know if you can get the correct amount of rows of colour for the height. I also used 2 batches of vanilla butter cake and equally divided the mix up with the scales before colouring my mix so each layer was cooked with the same amount and used 8" low silicon pans and cooked until it was not tachy to touch..around 18 mins 160. deg