Three tissue zones
Moving from the face inward: normal skin with hair follicles and sweat glands; then the vermilion — the red zone — which has a very thin, translucent keratin layer, no hair, no sweat and no oil glands, so its colour comes almost entirely from blood in the vessels beneath; then the wet line and the inner mucosa.
Lip blushing works on the vermilion. It cannot safely or usefully treat the wet mucosa, and it only feathers slightly onto the skin zone at the border.
Muscle and movement
The orbicularis oris is a sphincter muscle encircling the mouth. It never really rests — you speak, eat, drink and express with it constantly — and that perpetual motion is one reason implanted pigment breaks down faster here than on still skin.
Blood supply
Superior and inferior labial arteries, branches of the facial artery, form a ring just deep to the vermilion. They make lips heal fast and bleed and swell readily during a procedure — hence vasoconstrictors in secondary anaesthetics and the need for a light hand.
Landmarks
Cupid's bow, the philtral columns, the oral commissures and the vermilion border itself are the map points an artist uses. Ageing flattens the bow and blurs the border, which is exactly what a blush can visually restore.
