Your lips look red because their skin is almost see-through
Lip colour is not really pigment. The skin there has just three to five cell layers, against as many as sixteen elsewhere on the face, so in lighter-skinned people the blood vessels underneath show straight through. That same thinness, with no sweat or oil glands for protection, explains why lips crack in winter.
Most facial skin is shielded by hair, sweat and body oils, which keep it supple, discourage germs and help manage heat. Lip skin has none of these. It dries faster than the rest of the face and chaps easily. Its sparse melanin-producing cells in lighter complexions are what let the red show; in darker skin the lips carry more pigment and look darker. The lip marks the frontier where outer facial skin turns into the moist lining of the mouth, and why that change is so abrupt is still not fully settled. Better touch sensitivity, clearer facial expressions and a subtle difference between the sexes are all candidates.
Anatomy has named nearly every curve. The upper edge of the red zone is the Cupid's bow, the small fleshy bump at its centre is the procheilon, and the groove running up to the nose is the philtrum, from Latin for lower nose. The lips are packed with nerve endings and claim a large patch of the brain's sensory map, which is why toddlers explore unfamiliar objects by mouthing them.
Lips do a lot of mechanical work. They seal the mouth airtight, hold food and drink in, and narrow into a funnel that boosts suction, essential for a baby feeding at the breast and handy with a straw. They shape bilabial and labiodental consonants and round vowels, and they make whistling and playing the trumpet, flute or saxophone possible. Even hearing people use them: in the McGurk effect, what the lips appear to say changes the sound a listener perceives.
Across the animal kingdom, proper lips are mostly a mammal thing. In most vertebrates they are minor folds outside the jaws; only mammals have muscles attached, making them mobile. The naked mole-rat is an odd exception, its lips closing behind its front teeth rather than in front of them. Birds and turtles turned theirs into hard keratin beaks.
Source: Lip
