I get this almost every day from someone (usually older men) at work. It's a little gross, since there's that weird undertone of flirting/creepiness to it, emphasized by the fact it's 95% men who mention this to me. I love my henna, and I wouldn't stop using it, but I do wish I could stop the comments!
I do color in my eyebrows lightly so that they match a bit. I think that might be why people think it's natural, but do any other henna users have this issue?