So, how do you henna your hair?

 Deb wants to try to color her gray, or at least blend it in.  Dark brown and black hair synthetic hair dyes have PPD, para-phenylendiamine, and they can make a person very sick, so Deb wants to avoid that.  Henna will not color her gray as perfectly as a synthetic dye, but it will leave her hair in beautiful condition, and she won't risk sensitization to PPD's.