After seeing variations on these being posted repeatedly on the Baking LJ group I follow (bake x3) I eventually gave in and had a try at them.

A simple idea, take your basic cupcake batter and separate it into bowls and add a different colour to each bowl. Layer into the cupcake cases as desired.

I would have liked a more layered rainbow look, but my cupcake pans are pretty shallow so they were more randomly splodged in! I swirled them with a cocktail stick to give a more marbled look.

I overfilled the cases as you can see and some of them overflowed. Actually they’re not even cases at all as the world conspired against me and no shops had any when I was looking! So I greased the trays and lined them with a little circle of baking parchment, it semi worked, not wonderful.