http://www.hennaforhair.com/

I have been using henna on my hair for the last several years.  My hair has never been healthier and the color STAYS (basically you cannot get it out).  I know if you mix indigo in the with henna it will give you a dark brown color.  They have a book of how to's and also a lot of pictures with mixes.  It is not a quick process but I assure you the end result is gorgeous.  I highly recommend the company that runs this site.  They are local to me and she has the best quality henna you can buy.