Are you interested painting in 3D mode? Let's try the human mouth first. First, get the basic shade of the lips with a single color. Then, fade out the top side of the upper lip part and give it a less sharp edge. Then paint the depth area above the lips. Edge out the top lip and add more shades on the bottom one. Then add those small vertical lines with low opacity. Add a gradient map over it and another pink layer(color burn).The lips are ready and it will help you in your painting skills.