A client has asked for a tie dye cake. Thought I'd do it with my airbrush but it's too smooth and doesn't look right. Have seen photos with buttercream but no video tutorial. Don't want to do all the colours with fondant. Any thoughts? She sent me this as reference. I'm in the UK.

I just did a video tutorial recently. Practice first, but the trick is to use colors that are really contrasty so that you can see the different shades. If you use ones that are close to each other they'll run together and it won't give you the same effect. 

You can probably do this on buttercream too, just make sure the cake is really cold when you're working on it so that the buttercream doesn't get pulled off the cake. It will be harder to get the white on it. This kind of thing just works better with fondant.

Amazing, thank you. Will use my airbrush first and then use the spatula and paintbrush to create more of the tie die effect. Hopefully that will work.