Well, you're partly right about the drying,but it doesn't last long.  There is henna that will color your hair,there is indigo which will make your hair black if you use it alone, and there is cassia which will just give you shine and not change your color.  Catherine Cartwright-Jones has a site;www.hennaforhair.com full of info. and pics of people who did their hair(before and after shots),recipes of what they mixed.  You should browse her site,it is very good.  Depending of what you're looking for,I'm sure you'd find it there or at least a wealth of info. to  make an informed choice or choices. 
I use henna and indigo from her and I would never go back to commercial hair color again.  It is natural,but it your hair is longer it is helpful to have someone help you.  It's a day involvement,but take the time if you can for yourself.  
There are others here on the site that use henna,too so ask questions of me or them,I'm sure we all would be happy to share info.!!

Do tell when you've done it and if you like the look and feel.  There are so many recipes and even whip up your own.

There's alot of info. on that site!  Any questions,just ask!!!  I'm happy for offer suggestions. as I'm sure others are,too!