When learning how to draw lips we need to grasp their structure in comparison to one another. The top lip is always darkest as it is slightly tilted downward which gives less of a reflection at eye level. It's all dependent on the direction from which you view the lips but generally speaking, we should make the upper lip slightly darker. This is also because the bottom lip is rounder which gives it greater potential for reflecting light from any direction. This is a good tip for how to draw realistic lips in a mouth drawing.