Tie-dye makes me think of the psychedelic 60's and 70's.  Crazy, colorful, and carefree.  I remember making tie-dye shirts when I was in Girl Scouts.  I thought it was cool to be able to create something to wear myself.  

The tie-dye effect can be created using any colors you desire.  I think three colors work best.  Working with six colors was challenging.  I didn't like that the red started to harden before I had all the other colors on the cookie.  I wasn't able to get other colors into the legs and horn due the unicorn shape.  Simpler shapes (circles or squares) work better for this technique.  I used this cookie cutter to create this cookie.  I made all six colors of icing the red and purple were 15-second icing and all the other icings were flooded.  I piped the colors on the cookie and then used my scribe tool create the swirl effect.  Around the mane I tried to swirl the icing to look like a mane - it kind of worked.