Sorry - just seeing this. If the cookies are relatively freshly iced and the icing is not dry all the way through, you can easily scrape off the icing with a paring knife. But now that many hours have elapsed, that's likely not possible.
Airbrushing MIGHT work if the pink you used was not too dark. But water-based airbrush coloring applied over a dark pink is going to look quite different than if applied to white. The applied coloring is going to look darker.