Just thought I'd share really quick that the "Seven Pillars of Health" highly recommends that people don't color their hair. The chemicals are really bad for you...the smells and the stuff leaching into your scalp and entering your bloodstream. The book says that if you simply cannot go without coloring, go a shade or more lighter than your natural color. The darker the color, the worse it is for you. I can't find the specific pages tonight to quote anything, but I do know that it's best to just go gray (or in my case, white) naturally.
Personally speaking, I've been complimented many times on my hair, even by my hairdresser who has suggested I do not touch the color but leave as is because it's "very pretty." I'd rather see people with gray hair than to not see them at all, so please consider carefully if you really need to cover the gray.