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.
Did you ever eat icing that contained so much food coloring it tasted bitter? This is usually due to dark icing having so much food coloring in it. I have a solution though, instead of adding the color to white buttercream add it to chocolate buttercream. This will prevent that bitter taste that usually comes with adding a lot of food coloring. By starting off with chocolate you have a nice flavor and a dark color, therefore you will need less color so less bitterness. Just remember to tell your customers you are now using chocolate icing just in case they have any chocolate allergies!
Coloring icing is very easy to do with all of the food safe colors on the market today. There is basically every color you can imagine available in food dye. However it would be costly to buy all of them for the possibly of using them one day. I recommend buying the colors you need now and than mixing 2 or more colors to get the ones you do not have when you need them. Color mixing can be difficult to do, so I would start with mixing a small amount of color at a time. This way you will gradually see if you are achieving the color you need.