Kool-aid is an acid dye and will dye nylon. Nylon dyes like wool and silk, for some reason that I am sure a chemist would understand. I've kool-aid dyed Dappi pull-on nylon covers before, the nylon was lighter than my results with other fibers like silk or wool, I think because the nylon is coated to make it more waterproof. You could try kool-aid or acid dyes from Dharma. None of the other dyes will work though.
If you do a search on Pentel dye sticks, there are threads talking about using these on PUL like FBs. And a browse through the Dharma catalogue will turn up some fabric paints that work on nylon and polyester that might be worth some experimentation, I know there are wahms using fabric paints on PUL.
And as a last resort or maybe not, I bought a cover that a mama had doodled on with a Sharpie, and the doodle is still there.
