Well I have a friend where I used to work that does "tie dye" cakes all the time. Here's how she does it.
Pick the colors that you want on the cake.
then spray alternating colors on the cake (leave a white space in between the colors) and now for the magic, take a popsicle stick and (while the colors are still wet, won't work on crusted frosting) carefully drag the stick away from the center, now skip the width of the stick and do that again until you go all round the cake, Now in the spaces that you skipped go from the outside towards the center.
I know, a little time consuming but the end effect is kewl. Then I would write the message in 60's style bubble letters and you have your groovy "tiedye" cake
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