A lot of people use hair dyes (synthetics or the plant-based stuff like henna both) to cover grey hair. And a lot of times, people complain about how resistant grey hair is to taking up dye. Is there any truth to this, and if so, why might it be the case?

IS grey hair more dye-resistant, or is it just that the vivid absence of pigment means that the dye is already starting from less-than-zero and hence will turn out weaker?

IF grey hair is more dye-resistant, why might this be the case? I'd have imagined that, with NO pigment in the hair shaft to nuke, it seems to indicate that you should be able to dye it any color you pleased since it's almost like having your hair come right out of your head in stage two of a double-process color.

Any wisdom to impart, BB? Have there been any studies done of the efficacy of hair dyes or stains on grey hair as opposed to blond, red, black, or brown?