better late than never...
I have used Mountain Rose Herbs henna hair dye before, and loved it. https://www.mountainroseherbs.com/products/henna/profile
They have a lot of info on how the henna will react with your particular hair color, which is really helpful. I used a color called Sherry in the past, but it looks like they've changed their color scheme a little. I suppose I would go with red now over my medium brown hair. The color that I got before was a really lovely deep red color that looked pretty close to natural. Interestingly, my hair has retained quite a bit more red highlights than I had before, even after a year of having it in my hair. I just let it wash out naturally. I had a line in my hair for a while, but it wasn't terrible. They do say that henna will only cover about 10% of gray, though, so it isn't helpful if you've got a lot of gray that you're trying to cover. I'm thinking I'm going to do it again to my hair sometime soon. It was fun, and does help condition the hair. With the acidic water that we have it is really tough on my hair.