I have tried this image all three ways; all color, all black and white and spot color with just the lips. I like this version the best. It really draws the viewer' attention to her full and beautiful lips.
It could be that from a composition perspective you might be expecting to see the lips at that intersection of thirds? I think it has more to do with the subtle transition from chin to neck, and I believe if the shadow under the jawline is burned in just a touch it seems less empty without taking focus away from the lips.