I want it to come out after the first wash. This is just a prank that should not last more than 3 days. It could be chalk or dye.
Food coloring. That stuff that comes in a little box at the grocery store with I believe 4 bottles of color red blue green and yellow if I remember correctly. You mix them to make other colors. It's used for coloring frostings and dyeing Easter eggs. It comes out with one or two good shampoos.