I don't have a lot of experience with piping bags - the few times I've used them the icing hasn't been very good, but this time I allowed myself a lot of time to really get the buttercream to the right texture. The secret, I think, is to let it whip for a really long time until it's very light and fluffy.
I let the buttercream whip for about twenty minutes, and piped the swirls on really carefully, then sprinkled edible rainbow confetti on to it. I'm quite pleased with the results! The only downside was the patty pans - they were gorgeous rainbow patterns, but they were too thin, so because the cake was chocolate mud, when it cooked, the colour of the cupcake penetrated through the patty pans (which you can see in the top photo) and the colours were totally lost. Luckily the pack had heaps of extra patty pans, so I double layered them and all was fine.
I'm still working on how to get the cupcakes to cook in that perfectly round cupcake shape. I find that mine always go slightly too high in the middle, or a bit lopsided, rising more on one side, but overall I am super happy with how these turned out!