look up the Zone System. Zone 0 is pure black Zone X is pure white, and Zone V is middle (18%) gray. Caucasian skin would be normally a Zone VI.
I just took a "black and white craft" course with the most tech-heavy professor ever, so I've got all this stuff drilled into my head. Half the semester all we shot was blank white walls at different exposures, measured density, drew parametric graphs, etc. Haha. I could go on and on about black and white.