I didn't know this until I looked it up online. Another example of how much you can learn on this site.
The skin of the lip, with three to five cellular layers, is very thin compared to typical face skin, which has up to 16 layers. With light skin color, the lip skin contains fewer melanocytes (cells which produce melanin pigment, which give skin its color). Because of this, the blood vessels appear through the skin of the lips, which leads to their notable red coloring. With darker skin color this effect is less prominen. The skin of the lips is stratified squamous epithelium.
The mucous membrane is represented by a large area in the sensory cortex, and is therefore highly sensitive.
The skin on various parts of the body has different characteristics, different nerve endings, and purpose. The same is true on all animals with skin. Lips always have had slightly different texture and color, and "lipstick" is an artificial way to enhance this difference because the difference is found to be attractive or exciting. Lips have many different purposes..
Lip skin as well as other skin types on the human body vary in thickness. Lip skin is thinner - with more nerve tissue than the surrounding skin - which is why we don't kiss each other with our elbows.