I've striped a bag of icing before, but it was royal icing and for a very different project: piping roses with a striped edge. I wanted to see if the same effect would work for cupcakes (as you can see above, it did!). First, bake up a batch of yummy cupcakes. I really like liners by Cupcake Creations; check out how the white parts of the cup remain white, even when holding a dark chocolate cupcake (not all brands can pull that off).
Cuff the bag over your hand, folding the bag more or less in half.
…and how it turned out. I piped a smaller number of cupcakes with the blue, because blue is traditionally regarded as the most unappetizing color and I didn't want to be stuck with a surplus of them.