Both of the previous answers are not correct. In order to understand why the skin on the lips is different, one should start with embryogenesis and it will become clear that the lips are the first sphincter of the gastrointestinal tract (gastrointestinal tract), the gastrointestinal tract also ends with a sphincter called the anus. The gastrointestinal tract is lined with a mucous membrane, which has its own special structure and its cells that are well supplied with blood and therefore have a pink color. So in the process of evolution, it turned out that, as it were, an eversion of the gastrointestinal mucosa with the formation of a sphincter (circular muscle of the mouth) was transformed into what we now call lips. The skin on the lips is as close as possible to the structure of the skin on other parts of the body, but evolutionarily originated from other integumentary tissues of the body, therefore it has differences.
In the lip, there are skin, intermediate and mucous parts. In the transitional part, the connective tissue papillae protrude high into the epithelium, and numerous capillaries are contained. The blood flowing in them shines through a thin layer of the epithelium and gives this part of the lips a reddish color. The epithelium here is partially keratinized.
In fact, it is no different from the rest of the skin, it is just thinner, more tender and devoid of sebaceous glands, so that a person can better feel what he puts in his mouth, and for other important purposes.