I want to make a tie-dye effect on a sheet cake, but I don't have an air-brush. I've been debating several different ways of doing it. Any suggestions?

and this is on buttercream, not fondant

I've never done it but my guess is put the colors one next to the other and drag them with a damp brush just enough to combine them. HTH

You can go to Michael's and pick up a couple cans of edible spray (these are in the Wilton isle). I used the red and black to do this when i made my boyfriend Megadeth (Band) cake a couple of years ago and it turned out very good, they are very easy to control too.