If I got a dime for each time someone asked me about dyeing acrylic or cotton with food coloring/Kool Aid I would be a rich woman. I decided to answer these questions once and for all by creating a video showing people what you cannot dye with food coloring.
Surprisingly, the 100% organic (unbleached) cotton that I used in this tutorial did pick up some color. I am not sure how colorfast it is, but there are certainly more efficient ways to dye cotton vibrant colors. (Compare the cotton color to the 100% wool!)