I color (and bleach, gasp!) my hair. Yeah, it damages it. But I like the look and have still gotten some good growth.
As for henna and how much it will color your hair, it depends on how dark your hair is to begin with. If you have dark hair, you'll see a reddish tint in certain lights after henna-ing. If your hair is lighter, the henna will show up much more. Either way, if you have any white hairs in there, they'll come out bright red post-henna. I've experienced that and it looks dumb.