A touch of warm color in the shadow can be very effective:
I lost the freshness of the sketch by over-rendering the objects on top of the dresser and by not leaving more light valued shapes in the objects. To contrast the Payne’s Gray wash I created contrast with light valued, warm washes of scarlet red as cast shadows on the wall. Shadows are usually thought of as cool rather than warm. In this case, as in many cases, reality must be discarded for the sake of the painting or sketch.