Does anybody know how I could get a tie dye effect to my fondant?

Take small amounts of the different colors you want and kneaded and twist to getter to give a marbled effect like tie dye.

. . . this may offer some assistance to what you are looking to achieve . . . HTH

http://atecousa.net/learn/satin-ice-tutorial-iii-marblizing.html

If you have an airbrush, you can just spray it on. Those wilton spray colors probably just as well. You could also paint it on with color mixed with lemon extract/vodka