How Green Was My Hair
1. You should always read the directions on the black henna package, because sometimes the directions say things like "Do Not Use on Gray Hair." [nb: I have a lot of gray hair.]
2. If you put on black henna first, and it turns all your gray hair green, you can fix it by doing the whole thing again but with red henna, which will neutralize the green and turn it all something close to the lovely shiny auburn you were imagining. Actually, the relevant websites suggest doing something very much like this ad-hoc experimental solution for gray hair, only with the red henna first and the black henna as a 2nd step.
3. But if you try to use ONLY red henna, it will turn your gray hair bright, clown-like orange. [n.b.: actually I learned this part a couple of years ago. Hence the attempt at black henna.]
4. If you go in to work with hair that is partly regular brown, partly shiny auburn, and partly bright orange, only a few people will notice, or at least only a few people will say anything. [But I bet more people would've noticed if I'd had green hair!]
I just wish I'd taken a picture of the green part. It was kind of amazingly green. Maybe next time!