I think I first saw this on Martha Stewart's website? But I was reminded of it again on Pinterest. It's easy peasy. But still pretty. I used gel coloring so the effect is different then water based food coloring Both are pretty, I think the gel is more defined because it doesn't run. I also didn't want to take out piping tips. I did this all ghetto with a plastic bag sniped off at the end.
To make things easier I put the bag over a drinking glass. Strips of gel food coloring was stripped onto the side of the bags. Added the icing, snipped a hole in the bottom of the bag and pipped away.