Sure you can. (100% pure BAQ henna only) My first henna was over a salon brunette dye job that had about 1/2" of root growth--see May 2012 siggy-- (my temples were very gray because there isn't much hair there!). It came out great, no demarcation line, and left everything redder than before. I've not used chemical dyes since, but you can dye over henna with a darker color without problems. It will always flare red though. The trouble comes when you try to lift it and make your hair lighter. Then you're hosed. Henna is rock solid permanent. It won't fade or lighten with time, and you can't lighten it once it's in. No more bleach or highlights for you!
If you don't want to continue henna, you can; dye it darker, or grow it out and cut it off, transition to "glosses" rather than full hennas and ombre it out. I'm 47, and quite gray, but plan to henna for several years yet. My natural color is ash brown and gray now, but it was strawberry blonde until my mid-20's.
I also recommend a strand test before you jump in. (I collected 2" long sheds from the vanity top for several days in a coffee cup, hennaed them, wrapped in plastic wrap and carried in my bra 4 hours; rinsed in the coffee cup, and let oxidize in the bathroom for a week. Wowser! They came out red and sparkely and pretty! I've had that mud on my head every month since.) Let us know what you decide. Henna isn't for everyone, but I really love the stuff.
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