i would do a simple cake that you can split and color, rather than trying to mix two different textures - no guarantee it will cook at the same rate, and you don't want to end up with a weird texture (IMO)!

we make a simple vanilla cupcake the other day - just split it in two and add color. just keep in mind the red and yellow, if you swirl, will give lots of orange. maybe look at layers in the patty pan instead...