I have started to build up the tonal values using two of my favorite colors, French Ultramarine Blue and Sepia. They have such a nice texture together and depending on how much of each you add and how much water, a huge range of tonal values and hues. I continued adding thin layers until the tonal contrast was nearly completed on the face.
I then used a neutral gray for the background to add texture and interest. Sprinkling a bit of salt on the wet background paint adds additional texture and helps the pigments to separate giving interesting edges and a larger range of hues.