Is there any reason you don't want to use acrylics to coat the whole board with?
I'm using a midtone gray to coat the board (Amsterdam Standard Series - Neutral Gray). It's always that consistent mid-tone gray value, which makes it a hell of a lot easier to judge values than when I'd put them on a white background. No mixing needed, very quick to dry, easy to clean the brush with water and soap, and no issues going over the top with oils.
Also - there are apps on your phone that will tell you what color something is, might help you out