In fact, like painting in other places, it depends on the characteristics of the model. Let me take the mouth of "plaster David" (this should have been seen before) as an example. His upper lip is generally composed of three balls, and his lower lip is full. Draw a sense of volume. Draw the boundary line, reflect light (weak reflection), and just draw a few strokes in the bright part. . .

Other people's lips are all the same, but the thickness is different. Don't paint them too heavily. Lips are also skin. . . If the painting is black, it will be out of harmony. The cleft of the mouth (where the upper lip meets the lower lip) seems to be a black line. A thin reflection will contribute to the sense of volume ~