I've been researching this for quite some time and there is so much information and misinformation on the web. The one true sight I have found is Mehandi . There is a ton of great information here and links to other sights that are helpful. Another great resource is Henna for Hair. Anything you need to know is right here.
So what hyped me up you say to try this...get this: I'm a case manager for a community mental health center and I went to go and visit a family of mine where the children are bi-racial Arabic and Caucasian. So I couldn't help but notice how shiny and pretty the young girls hair were and it had the most beautiful red tint! So I asked the mother if their hair was naturally reddish like that (mom is Caucasian and a red head) and she laughed and said "no, I henna their hair". Of course this turned into a 20 minute conversation! The mother then proceeds to pull out a carry-on bag filled with henna! She told me she got it from Iraq the last time they went. She then informed me that her mother-in-law told her to brew some tea and mix the henna with that. The mom said that's all she does and she does this to their hair once a month. So now.....I'm convinced and I had to try it!
Today, I exchanged the henna I had when I noticed it expired last year and bought Nehal Herbal Mehandi henna. It is henna with harad, bahera, amla, shikakai, coffee, kattha, brahmi, and Bhringraj in it. I also ordered some Rajisthani henna from Mehandi. I'm using the Nehal Herbal because I could get it locally and it should be okay until I get the henna I ordered. I'm going to mix it with the tea and see what happens. But I want to find a mix that gives me exactly what I think I'm looking for (since I've never henna'd).
Peace