If you mean a synthetic-free permanent hair colorant, the best one is henna. It's got a bad rep, but mustly because people rarely to never used the real stuff and only used stuff that mixed some henna in with metallic salts that would fry your hair up. Pure ground-up henna leaves, the stuff that's used by good mehndi artists, is great for hair if you want a certain spectrum of colors. It should however be considered permanent. People often talk about their magic special formulas to remove henna, but I'm a skeptic, and you shouldn't count on being one of the people that supposedly can lighten henna.

Other plant-bsed hair dyes that people use are indigo (gives either brown or black when used together with henna), cassia (gold tones, good for grey-haired women who want to stay blonde), and buxus (ash-brown tones). You can mix them together in various proportions to give a decent palette of colors. However, NONE of them will lighten your hair. If you are brown and want to go blonde, the only alternative for you is bleach or peroxide. All of the supposed "natural" solutions act by creating low levels of peroxide anyhow.

A good place to read up on henna mixes is on hennaforhair.com -- they have a downloadable e-book that will outline a lot of this. They also sell overseas, but I don't know the vagaries of international shipping very well.

In general, henna, indigo, cassia, or buxus are good plant-bsed, non-synthetic hair dyes. They are PERMANENT. You must also be sure you are getting the pure product and not some boxed garbage or else your hair may suffer greatly. And they provide no lightening at all but only add tones to your existing color.

The best way to control the end result for henna is by using a test hairball of hair harvested from your brush or comb. And to use less than you think and work up to a color buildup rather than trying to get the whole color in one go.

Body-art quality henna (a green powder composed of ONLY henna leaves) is fine for color-treated hair. It won't turn frog-ass green or anything on you. But definitely do a hairball test. The site I mentioned above sends out cheap samples for this purpose, and the e-book on the site is a great resource.