To keep your colour looking great, WITHOUT DAMAGING your hair, there is a mousse in temporary colour refresher that, though it is not made with natural ingredients, is the closest thing to perfection I have ever found. You can see from the ratings and reviews, other people agree with me.
Goldwell's Colorance Soft Color
Goldwell Colorance Soft Color Foam Colorant Mousse - hair color
Goldwell Colorance Soft Color Foam Colorant by Goldwell, Hair Color Review
If you really want natural products, you can henna your hair. The henna plant produces an orange pigment. You can add Indigo, a blue plant pigment to this and get a brown colour. This process is arduous, but well worth it.
I'm a redhead, so I go with only the henna, but seeing a girlfriends results with a rich chestnut brown she gets by adding the Lush brown henna (which contains indigo), to a 100% henna, I am convinced this is what I would do if I needed brown hair. Her results wet stunning! The only issue is that, depending on your texture, you very may well loosen your curls. It did happen to me.
For quick great results, but with chemicals, and no loosening of curl, go with the goldwell!