I have seen these products in the local health food shop and had a good read of the website. They seem to fit the bill perfectly as I like you, am just about to embark on covering some minor greying.
The website is really useful, read every tab and I think you'll find everything you need to know about the products. I'm just trying to decide now, which colour to buy, its not easy for a first timer! Let me know what you think as I have medium to dark hair too.
http://www.herbatint.co.uk