When used as a hair dye, henna is a natural way to colour your hair, with a range from black to red to brown. Henna works by coating the hair strands; the use of heat is essential in a henna treatment as it basically bakes the henna into your hair's pores. Thus, it is generally the case that those with fine hair will get less of an effect and will not last as long as those with coarse and/or curly hair. It's been said that henna cannot lighten your hair, but in my own personal experience, I'd have to disagree. I myself have curly hair, and as a child had this kind of neon strawberry shortcake red hair, which faded to auburn as I matured. In college, in a typical female fit of drastic hairstyle changes resulting from boy troubles, I hennaed my hair 'Natural Expressions' red (The boxed henna found in your typical Wild Oats in the states), which resulted in the most pleasant bright red you can imagine. It was significantly lighter than my natural auburn. I refined this over time by combining mahogony and red, a cool thing you can do with henna. The point here is not to just slap it on and hope for the best... as tedious as it is, do a swatch test before you layer your head in the stuff, just like the instructions say. Don't blow it off, do it, or roll the dice.
You can buy henna at just about any wholefoods store, or a LUSH shop. It tends to be cheaper than chemical dyes, and is much less stressing on the hair, to a point. If you have long hair, the buildup of henna will be significant over time, and depending on how drastic the color change you're producing, a bitch to stop using. Also, if you have curly hair, over time it can take quite a bit of the curl out... you may find this to be a good or bad thing, depending on how much curly hair angst you have... if any.
When preparing henna, be ready for a mess. No, really, it is a colossal mess. Natural henna has the consistency and appearance of horse shit - kinda brownish green with little sticks in it. It smells only marginally better. When you mix it, use a pyrex bowl and wooden spoon. Do not use any metal objects 'cause this will funktify your mix. You can add cinnamon to the mix, which is nice, and helps auburns and reds. The best thing to do is take your bowl into the tub, flip your head over, and proceed to squelch it through. Be sure to wipe any of the mix that glops off anywhere, 'cause this stuff stains.
Okay, so at this point you'll be looking like a mythical beast, I wouldn't suggest accepting any calls from beaus you're wishing to impress... Pile your hair up (if need be) and cover your head with a plastic bag... Now this is the fun part. You have to keep your head warm for between thirty minutes to an hour. You can use a hair dryer, heat lamp, some summer sun, or towels warmed in the dryer that you don't care about. Follow the directions, and check it as you go along by rinsing a strand and checking the color. Be sure to dry it as well, to make absolutely certain you're going in the right direction. If you're not happy, recover and shove it back with the rest of the bird's nest. Have paper towels handy for drips.
When you're happy with the colour, rinse out the henna. This will take forever, mind you. Take a brush with you into the shower that you don't care about to help get the little sticks out of it. When you're done, don't condition it or anything, 'cause your hair will be very very soft and naturally conditioned by the henna. Your hair will smell rather earthy if you didn't add cinnamon, but it won't smell bad.
Now.. for those of you that chose red... go out and enjoy all the rubber-necking lads who believe all those things they say about red heads