I have been coloring my own hair for many years now, it started to go grey when I was in my twenties and I have used most of the top brands with a good rate of success. I have never used a natural hair color to cover the grey, so I did some research on Google to see what others have to say about these products and to check out what is available.

There are not really many options in natural products to cover grey hair, but I found one, lotuspress.com who features Light Mountain hair color, this is what they say about it;

"Using henna on gray hair has always been possible, but somewhat difficult. Henna has a harder time attaching to gray hair because of changes in the protein base of gray hair. After years of research, Light Mountain has developed a two-step product, Color the Gray! that effectively colors most gray hair. Available in eight shades, our color chart will help you determine which shade is appropriate for you. Inside the box are two treatments: Step 1 is applied first to condition the gray hair, thus aiding the acceptance of the final color treatment. Step 2, the final treatment, seals in the color. Color the Gray! is the first and only natural hair coloring treatment system for gray hair that does not rely on any chemical agents.

The premier natural hair color and conditioner product line. Choose from our original line with the famous "henna animals" in 12 shades, or our unique Color the Gray line with 8 shades. We use no chemicals and no synthetic ingredients, only pure premium henna and other botanicals. We have been blending henna based natural hair colors since the early 1980's and are the leading totally natural line in the natural products industry."

There is another site, care to.com, which has some information on a couple of other brands, this is what they say;

"We tested the gray-covering ability of Herbatint and Naturcolor, two brands that contain no ammonia or resorcinol and a minimal amount of peroxide. They both provided excellent coverage and left a natural-looking color that lasted for months—and the fewer times the hair was shampooed, the longer the color endured."

These brands may be worth trying, I hope this helps.

Depending on your natural color, henna is a great natural hair dye.

http://www.hennaforhair.com/faq/index.html

Henna is actually the best possible natural dye for grey hair. The thing is, you have to use body art quality henna, or you will end up with a tangerine head of hair - or worse, no effect at all. So make sure when you buy the henna, you are getting a good quality henna typically used for henna tattoos and body art.