2 packets boxed vanilla cake mix (made as per box instructions)
6 extra eggs
Gel food colourings
Sprinkles, to decorate
Preheat the oven to 190C (170 C fan). Line 4 Swiss roll tins with lightly greased foil, and pinch a raised fold halfway across, to split the tin into two longer strips.
In a large mixing bowl, whisk the boxed cake mixes with the ingredients as listed in the instructions on the box, along with 6 extra eggs. Whisk until light and fluffy. Your mixture will weigh around 2 kilos, and each tin will take 500 grams in its 2 segments, so divide the mixture into the right amount of bowls and colour them as you wish. We used 6 bowls with different amounts of mixture in each: 500g purple, 500g blue, 500g green, 250g yellow, 150g orange, 100g red
Tip your coloured batters into the foil segments. For the middle of the cake you can put red and yellow into the same segment, using a spoon to get a straight edge between the two colours. Spread the batters out smoothly then bake for 10-12 minutes until firm to the touch.
When the sponges have fully cooled, tip them onto baking paper and, starting from the middle (red), fill with buttercream and roll them up - taking out the baking paper as you go. Once the cake is assembled, transfer it to a serving plate or stand and coat it a crumb coating of buttercream. Let this chill for 2 hours, then coat with another layer of buttercream and top with sprinkles, then serve.