I use to be a hair dresser (add all those toxins to my mercury <img src="/ubbthreads/images/graemlins/frown.gif" alt="" />) so I read alot about hair products and know that they say that the safest hair color is a semi-permanent color as it doesn't contain as much bad stuff and penitrate your scalp as much. Semi-permanent last 6-8 weeks depending on how many times you shampoo of course. The only non chemical hair color is natrual henna which doesn't cover grey.

One other option would be to do a reverse highlight-pull strands through a cap and apply color to the strands only. This is safer because the color doesn't make contact with your scalp.