I'm no henna expert, so everything I'm going to sday is based on what I've read the monstrous henna thread.
First off, in order for henna to color the hair, the dye needs to be released with what's know as a terp. Something acidic like lemons or ACV. There are others, but I can't recall. You also need to let it sit for about 12-16 hours for the dye to release once mixed. Heat is also another factor that will cause the dye to release.
SO here's how I've been using it. I simply mix the henna into a paste with cold water. Add conditioner and amla oil and I'm done. I apply it right away, place a plastic cap on my head and let it sit for an hour or two. That is it. I think this method is referred to as a henna glaze. WHich is a procedure that provides the benefits of strengthening and conditioning the hair without coloring.
With all of that said. There was one person in that thread who stated she did this and it turned her sister's hair red. Therefore it's very important to get information here as to which brands others have used with success. One other thing worth noting is that henna will probably always deposit some type of color into the hair, howver faint. But it cannot turn black hair red. My daughter and I both have black hair. I did find a strand of grey after I hennaed and noticed it had a slight tint to it. Nothing drastic, but it was there.
Also, I think it's really important when starting out to start with the basics. Which to me is 100% lawsonia (henna) (not mixed with anything) and then add a few conditioing agents to it. As you become more comfortable, then you can start to play with your mixtures.
ANd last but not least, always do a harvest test. I didn't do it, because I'm a daredevil like that
I was pretty sure that nothing drastic would happen. But I do believe it's an important step. I was just too impatient. I wanted to get the henna in my hair.
Well good luck and I hope this helps you.