Start by greasing 6 non-stick 6" cake tins with butter or oil and line them with baking paper if necessary.
Next, add 700g of room temperature butter to a large bowl.
Add 700g of sifted caster sugar.
Whisk for 3-5 minutes or until pale yellow.
Add 1 tsp of vanilla extract to the mixture. Then measure 700g of room temperature eggs. Add one into the bowl and whisk until the egg is no longer visible. Repeat the previous step for the remaining eggs. If you add all of the eggs at once the mixture may split and curdle. The mixture should look smooth and silky.
Add 350g of sifted self-raising flour to the mixture and 2-3 tbsp of room temperature milk and mix. Repeat the previous step with the remaining 350g of self-raising flour and milk then mix.
You will need red, orange, yellow, green, blue and violet food colouring. I suggest you use gel as the colours come out a lot more vibrant. Add equal amounts of the cake batter to six different bowls and add 1/8-1/4 tsp of each food colouring and mix until well combined.
Add each colored cake batter into the greased 6" tins and bake for 30 minutes or until done. To test if your cake is ready stick a knife into the cake. If it comes out clean then it is done but if it is wet with cake batter then it's not ready. Once the cakes are ready leave them to cool completely on a cooling rack before decorating.