Fnord wrote:
My lovely wife is Asian, and her hair is naturally black, although lately it seems to have acquired a few reddish-brown highlights.
She keeps a few bottles of henna rinse. "Reshma" brand, I think...
The reddish-brown highlights are probably greys that were rinsed with the henna.

I've used a henna-indigo mix before, and it gives a reddish brown tint. Pure henna gives a bright red tint to greys. Both make a pretty color!