Cassia Obovata is the name of the neutral henna that does not color your hair. (Of course, I can't speak for the people who have used it and say they had other results...haven't read enough about that yet...but Cassia should contain no dye.) Check it out here:
http://www.hennaforhair.com/samples/
Henna should be perfectly fine to use over chemically dyed hair, or permed hair, etc, as long as it is body art quality, but always do a strand test first as results vary so much for everyone.
You can put essential oils in your henna mix to make it smell better, also.
And yes hennas do come pre-mixed for certain colors, but to make sure that you know what's in it, it's best to mix it yourself. It's not hard to learn to do. Much of the henna you find in stores, even health food stores, does NOT have all ingredients listed on the packaging, and then you risk having mineral salts in it, which mess up the color of your hair.
As with any other dye it's important to do a strand test before using henna to know how it will react with your hair. Also, henna isn't like every other dye, in the way that it takes hours to work. Maybe 2, or 3, or as long as 8. You should be prepared to spend a day with it on your head, not to mention the time it takes for the dye to release after you've mixed the henna. This can take up to a full day.
If anyone wants to know how to do this and gets confused, I can let you know at least some of the things to put in your henna to get different results...but it's all on the site mentioned above.
Many people who mess up doing henna simply did not do a strand test or read up first on what they should put in their henna to get their desired results. If you do those things, you should be fine.