As I needed to fill 3 tins I doubled a Victoria sponge mix. The mix was divided in 3 and I added yellow to one section and orange to another. I found at all a bit messy as the divider had to be washed between filling each tin.
As you can see it did not turn out like the picture on the box.
I think the only way I am going to get perfect squares is to bake 3 coloured sponges and use the divider to cut them. It is such a big cake that, that experiment is going to have to wait until there is another special occasion.