The lips and genitals have a different natural pigmentation than the rest of the skin due to the presence of specialized cells called mucosal cells. Mucosal cells contain a type of pigment called melanin, which gives the skin its color. However, the melanin produced by mucosal cells is different from the melanin produced by skin cells, and it results in a different color.
The lips, for example, have a high concentration of mucosal cells, which contain a smaller amount of melanin than skin cells. This results in a pink or red color to the lips. The genitals also have a high concentration of mucosal cells, which can produce varying shades of brown, depending on the individual's skin tone.
Additionally, the skin on the lips and genitals is thinner and more delicate than the skin on the rest of the body, which can also contribute to differences in pigmentation. The lips and genitals are also areas that are less exposed to UV radiation from the sun, which can further contribute to differences in pigmentation.