I've also seen one where you carefully add the new color to the center, and it pools out in rings. I'll see if I can find a link to a recipe/pic.
Here's one...I would skip the black stripes. (This one is rainbow zebra.)
https://mycupcakeaddictionblog.com/2...ow-zebra-cake/
Another option is to buy a Funfetti cake where you just stir in rainbow sprinkles.
Rainbow polka dots! That's cheating. I know.