Another great technical Vader topic!
Part of the mystique and menace of Vader to the movie viewer is not being able to clearly discern his facial characteristics. Many are likely unaware of the two-tone paint scheme. Multiple indirect light sources from various directions and of varying light source dimensions (point, line, and plane) make it difficult to discern the surface details of the mask. And it also makes it difficult to determine what's an diffuse reflection off a surface painted gray versus a specular reflection off a surface painted gloss black (unless, of course, you already know the paint scheme).
Irvin Kershner was masterful in his use of lighting in ESB. He made Vader look good! As he put it, ''As a former cameraman, I have very, very definite ideas of how to stage for the camera, and lighting. I chose my own cameraman. We worked together on making it look darker and let's say more mysterious.'' Ah, what a different movie ROTJ could have been with him at the helm. And I doubt Vader would have had that retinal scanner spotlight shining in his right eye as in the Endor corridor scene!
I did some web searching and found a discussion on "Light Reflectance Value" or LRV which is a measure of the lightness or darkness of a color. It's apparently on the back of some paint swatches.
http://thelandofcolor.com/lrv-light-reflectance-value-of-paint-colors/ This might be useful as you evaluate and select paints for future projects.