Blood flow. <– Redder lips
Dead tissue. <– dry lips
Notice sometimes that when you put lip balm or chapstick on, the skin that was dry still seems to flap and be dead?
Yes, that’s because it is dead.
A way I read in a book for a woman to increase the redness of her lips is to bite into them which would increase blood flow so that they would become more plump and red.
If I remember correctly, as RBCs pick up more oxygen the hemoglobin becomes oxyhemoglobin which gives the hemoglobin RBC structure a red tint. Thus I assume that more RBCs coming to the surface will enhance the redness.
Most lip balms are made from mineral oils(petrolatum) and carnauba wax(stuff that forms cuticles on leaves). I’d think it’ll have the same effect, lessening exposure to air and evaporation of moisture. Might make you lick your lips though 😛