If you don't want the red that henna gives you, you can mix it with indigo to get more of a brown. You may also want to consider trying a henna gloss or hennindigo gloss, which is easier to maintain and not as intense as straight henna. You can read about these options at hennaforhair.com.
I have never tried henna or henna glosses because I don't think I would look good with red hair and I'm too scared to try to get a rich brown because results vary a lot, and are permanent.
I have light to medium brown hair and a small amount of gray. I use Cassia and Chamomile to condition my hair and to dye my grays a golden color so they blend in. The color isn't usually permanent like the henna is, so I repeat the treatment every 2-4 weeks, ideally.
I just brew some strong chamomile tea, mix it with some Cassia to the consistency of pancake batter, get in the shower and wash my hair... then I apply the leftover chamomile tea to my hair, squeeze it out and apply the cassia mix. I put my hair in a shower cap and then wrap an old towel around it and try to wait 3 hours before washing it out. (Some people report getting color in an hour or two, but I feel like if I'm going to all that trouble, I may as well leave it on as long as I can stand it.)
Some people just rinse it with water and/or conditioner, but I rinse it and wash it with Aubrey Organics Egyptian Henna shampoo and condition it with the A.O. GPB conditioner and then add a little bit of emu oil while it's still damp. My hair ends up feeling like silk.
You can buy cassia obavata from the hennaforhair site, or buy Senna powder from Herbco which works just as well if not better and is much cheaper.
You can read all about it on this thread at the long hair community:
http://forums.longhaircommunity.com/...read.php?t=406