I saw some cupcakes like this on a blog somewhere and I'm sorry I can't remember where it was! I was deep in blogger world looking for some ideas for birthdays and came across some rainbow/tie dyed cupcakes.
Start with the batch of batter and separate into small bowls, add the colors you want to use.
Spoon out a scoop of each color into the baking cups and keep adding colors till they are full enough.
use a toothpick to swirl the colors in the middle
You don't see them frosted because I let my son experiment with the frosting and they turned out a funky brownish blue color. not so cute :)
If this idea was on your blog please let me know so I can give you credit!