Airbrushing cakes is one of the easiest ways to decorate. To successfully get the look of Tie dye on buttercream,I suggest first putting your cake on a turntable and having your icing as smooth as possible. Now choose the colors you plan on airbrushing with and start off with the lightest color first. I always start with yellow. I make a thin w pattern in a circular (swirl) motion around my cake with the yellow, once I complete this I start with my next color leaving a tiny bit of white in between the yellow and the color I am using. I continue this until I use all of the colors I planned to and my cake is completely covered in Tye dye. It is okay if your colors blended together not all Tye dye is the same. You can also use white airbrush to lighten up areas where you would like it to be a little lighter. My cakes are usually bright tie dyes as I recommend against pastels because the effect does not come out the same.