Kool-Aid Hair Dye

heehee found that link while I was looking. It is for dying wool and fiber but it may get you going - just don't put your head in the microwave! Salt & Vinegar seem to be the agents to make it work - plus heating it.
In a book I just read, they were discussing hair dyes and the things they used were henna for red and black walnuts to darken the hair. No idea how it would be done but black walnuts are really strong and even stain your skin so I imagine it would be pretty easy to darken your hair with an extract made from the boiled shells.

Have fun!