Do a test!
The first time I hennaed years ago, I wanted to be a redhead, not just strawberry blond, but I was worried about the outcome. My hair was down past my shoulders so I did a test strand in the back underneath close to, but not on the hairline. I figured if I hated it, it would be pretty well hidden, even if I wore my hair up. So for a week, I had light strawberry blond hair with one red curl that no one noticed. But I loved it and went back and redid all of my hair. Since I am using a different henna this time, even as a gloss, I may do that again.
I have used the cassia as well. It just made my hair shiny, with no visible color change. But I got it at Sally's and it was labeled as "clear" henna so I don't know about the quality of it. It did not last nearly as long as henna; I remember that.
RCW - I read on a website last night that henna can dry your hair out. I've never experienced that. Have you? I am looking into oils to add to my gloss directions while I wait for my henna to arrive. I am impatient!!!