I got a high-quality model skull over the holidays so it only seemed right to paint it a couple times. This has been a very effective form of practice for me, using a limited palette of raw umber, titanium white, ultramarine blue, cad. yellow light, yellow ochre, cad. red light, and burnt sienna for the drawing. This allows me to focus on value while adjusting temperature slowly and focus on other things like edges (esp. in the skull on the left).
