Postscript: Since I first wrote this tutorial more and more gray hairs have grown in. Because of this when I henna my hair about an half inch to one inch of my hair (mostly at the temples and also long the hairline on my forehead are ending up bright orange. This is not to my liking. But, I've found a solution. You can follow up your henna treatment with a mini indigo treatment. This can be done no longer than two weeks after your henna treatment.
Tame Firey Orange Henna Color with Followup Indigo TreatmentIndigo applied to henna-treated hair strands will give you a richer auburn, shades of brown, or black hair. The longer you keep the indigo paste in your hair, the darker your hair will turn out.
Warning: Don't indigo your hair without first doing a henna treatment. If I were to apply indigo to my virgin gray hairs (wow, virgin gray sounds like an oxymoron!) I would end up with blue hairs. No thanks!
Mix 1/2 cup hair conditioner with 1 to 1 and a half tablespoons of indigo powder and allow to set for ten to fifteen minutes. It will be a slightly gritty paste mixture. Apply with gloves to the orangey portion of your previously henna treated locks that you wish to tame down. Cover with plastic wrap. Allow to set 15 minutes for auburn, 20-50 minutes for brown, and an hour or longer for black hair. Twenty minutes is my magic number for rich medium brown hair with reddish highlights. Rinse out thoroughly. Blot dampened hair with a towel and allow your hair to dry naturally (avoid using a hair dryer, the dry heat brings out more red). Do not shampoo your hair for at least 48 hours, allowing the indigo to oxidize.
Tip: It has been suggested that a couple of shakes of the salt shaker can be added to the indigo paste before applying to your hair if you've discovered your hair doesn't absorb the color very well. I didn't have that issue, but perhaps this tip will help someone.