This cake was made with coloured shapes of butter cake inside.  

When making the cake I portioned the batter into 3 bowls, added primary colours Red, Green and Blue to the bowls and mixed that through the batter to give me bright colours.  

Then I used silicon molds, one of squares, one of triangles and one of round balls. I added the batter to each mould and baked them at 150°c to help keep the vibrant colours.  After baking off the shapes I allowed them to cool, then when making the plain vanilla cake I added a little yellow colour to the base cake and poured it into my tins.  Then I took a few of the coloured shapes and pushed them into the cake batter dotting them around the cake evenly.  Then I baked the whole cake. 

When the cake is cut you can see the shapes inside but in hindsight, I probably should have not bothered with the triangles you couldn't really see that they were a different shape to the squares when cut. ;-)  Although this cake was delicious of course (Basic Vanilla Butter Cake) and everyone loved it.  The kids and adults loved the bright colours inside.