Just going layer by layer, darker and darker, careful not to cover the light highlights of the paper showing through. Notice I made the model's arm's skin tone a slightly different color than the rest of her skin - more blue (which looks greyish).
I drew a design pattern on her arm with pencil lead, just random, and began to paint it with a very fine brush. I used a random mixture of paint colors to make "black".
The background looks grey in these photos but it is really pure white. I'm thinking of painting in a graded background while I place a piece of tracing paper over the image and color it in with a pencil and smudge it with my finger. I still have to finish painting the mask itself also.