I just thought that I would throw in the idea of Elumen by Goldwell its an awesome prodouct with no amonia.  Has great grey coverage if mixed correctly, and I've found it to be a great product for people that have sensitive skin and or allergies.  However it has great staining effects so I wouldn't recommend using it on someone that likes to change their hair colour often.  And it stains the skin quite a bit.   But it is by far my favorite product.