The top lip is usually darker because it tilts away from overhead light.
The human face follows surprisingly consistent mathematical rules. Secrets to Drawing Realistic Faces
Use a blending stump or tissue, but don't overdo it, or the drawing will look "muddy." The top lip is usually darker because it
Real skin isn't perfectly smooth; it has pores, wrinkles, and imperfections. but don't overdo it
Use a mechanical pencil or a "stippling" motion for subtle skin texture.
Avoid harsh borders around the nose or lips; use shading to define edges.
Drawing a realistic face is less about "talent" and more about training your eyes to see shapes rather than symbols. Most beginners fail because they draw what they think an eye looks like, rather than what is actually there. Master the Proportions
