My friend's family is Hindi and they use henna hair dye a lot, and it always looks good. I braid her hair frequently, so I know it isn't damaged at all, and I also know that it DOES work well on dark hair. Whether or not the hair will brighten depends on the color. When she uses red/orange/hazel hues, it does tend to get a little brighter, but when she uses the blue or black it tends to stay the same or go slightly darker. Brown either lightens her hair or darkens it depending on the mix.
The initial dying takes a lot longer than with unnatural products - from 30 minutes up to four hours depending on how intense you want it!
As per maintenance, it is pretty low as hair dye goes especially if you are going for just a half shade lighter. The closer it is to natural, the less likely your roots will show and it will be noticed as your hair grows out. Most people who use henna, like people who get hair proffesionally dyed, will increase its longevity by getting a shampoo and conditioner like loreal's and revelon that deposite color as you go. Overall upkeep as it is mild tends to be once every eight weeks, so it is the same as demi-permenent dye ifyou want to keep the same vibrency. But again, this is dependent on how thick your individual strands of hair are, how MUCH hair you have, and how quickly your hair grows. It could be you only dye your hair twice a year.
A really good site is hennaforhair.com - you get an idea what different mixes do for different people and what would look best for you.
It'sjust natural hair dye and over all, it doesn't leave my hair dry, I do touch ups on my hair because I lighten it when I feel like it, it lasts up to 6 months, not at all high maintenaince. There usually is the colour on the box to match your desired hair colour to.;*)
I have never used it to lighten my hair so I'm afraid, I don't know about that, but I know it lasts quite a long time. I've used 'herbatint' it's a brand in natural health food stores. ^_*
I am a cosmetologist and henna hair dyes do not lighten and you have to keep reapplying them... the thing with them is that it just keeps getting darker and darker every time you use it... they are not a good idea because if after you use them you ever use regular hair color then you could have some bad consequences... I had a lady come into the shop that lied about using them and when we prelightened her hair it fell off at the scalp and started smoking under the dryer, and they are a bad idea... the best thing to do is prelighten your hair with bleach and wash that out then choose the color that you want to put on and put it over your prelightened hair...
Make sure you want it for a long time. After the first time I dyed my hair with henna I still had traces of it left over three years later; even with regular trimming and a few attempts to change the color again. It worked wonderfully for the effect that I wanted at the time, it hardly dried out my hair at all and required little to no maintenance. Only down side I experienced was it lasting a little too long for my preference.