The mouth – when drawing the lips make sure that you draw the line which separates them – it lies on the semi-cylindrical surface of the jaw bones and follows the rules of perspective stated below. The upper lip is usually thinner then the lower. There are 3 groups of muscles, one on the upper lip and other 2 on the lower one. First construct them and then put the shading following the form. Usually, we shade the upper lip and we put the shadow under the lower lip. Add some light or don’t put color on the middle part of the lower lip. Add some accents of dark shades in the corner of the lips and between the lips.
“Portraits.Faces.and.Figures” by Giovanni Civardi
“Drawing the Head and Hands” by Andrew Loomis