~Maybe~ on the gray.
I come from a custom photofinishing lab background, and i've found gray primers are close to something we called 18% gray. That's a gray used in backdrops we used that would neither absorb nor repel (ie: alter) colors and density (light/darkness). 18% gray is a perfect balance in PHOTOGRAPHY, but with paints it seems to differ.....and I think that has to do with the density (micas/flakes of solids) suspended IN the paint.