I got this information on Henna from Pennie at Ladies Blend..
Update from Natural Henna for Hair post.
I sent an email to Henna Expert Nakia Dawkins asking if henna would cover gray hair, with her permission I am sharing her response:
I'd be happy to share with you any information I have.
Henna can cover gray hair. The thing is henna is cumulative on hair meaning it gets deeper and richer in color with each application. Because it does not strip hair as traditional dyes do there is no danger of hair damage with consecutive applications. Henna is actually very good for hair strengthening it. Henna causes gray hair to turn a auburn/reddish hue giving your head of hair lovely reddish highlights. I recommend using only PURE henna...not henna hair dye.
There is a difference. Most of the boxes of "henna"; found in the beauty section of the store with hair dye is chemicals mixed with a wee bit of henna. This is not natural and I can not attest to the health of ones hair if they choose this method.
If you choose to use pure henna, get a box (I would suggest using Jamila as it has pretty consistent dye quality) You mix the henna with lemon juice, or yogurt and slather this goop all over your hair, working in sections from root to tip. It looks rather like green mud. (The paste should end up the consistency of yogurt) cover with a plastic cap or plastic wrap and allow to sit. The longer you let this sit on your hair the better your hair color. 2 -3 hours should be sufficient. Be sure to wipe away any splatters right away or you'll end up with a hennaed forehead...lol.
When you rinse this out you may notice a lot of difference or relative little difference depending on your hair texture and color. For most people it makes the hair softer and stronger and gives it an auburn hue. It will not cover gray as quickly or totally as commercial dyes , it will not dye your hair black or blond or any other color other than the lovely rich henna color.
Other quick pluses to henna
*** it is a 100% natural total sunblock
***It strengthens hair
*** It is cooling and soothing to the skin
HTH's and let me know if you have further questions
Nakia not only replied back promptly, she is also very kind and willing to share her knowledge on the subject of henna, which I am truly grateful and thankful for. I have never had to color my hair, but when the time comes I feel more prepared to experiment with henna, I think my hair would do well with it.
From Mountain Rose Herbs
Henna is a natural plant coloring for the hair; made from the powdered leaves of a desert shrub plant, Lawsonia. The Henna we offer is 100% natural and organically grown, and is free from pesticide residues, with no synthetic additives or artificial coloring agents.
Henna contains hannatannic acid which, when mixed with hot water, will coat the hair. It seals in oils and tightens the hair cuticle giving your hair a rich, healthy shine. Henna has no lightening action, so the shade that you choose will depend on your hair color (natural or tinted) and it must be noted that Henna will not work as a colorant if your hair has more than 10% grey. Its effects will last up to 3 months. Instructions are included with each order of Henna.
Short fine hair requires: 2-3 oz. Medium length hair: 3-5 oz. Long hair: 5-8 oz.
Choose from: Red, Black, Sherry, Mahogany, Burgundy, Dark Brown, Medium Brown, Light Brown, Copper, Marigold Blonde, Neutral, or Strawberry