为什么我们看到的嘴唇是红色的？因为它的表皮很薄、非常柔软，而且是透明的，所以，我们就能看到嘴唇表皮里的血的颜色。而且嘴唇下的毛细血管特别丰富，感觉敏锐，所以人总是喜欢用嘴亲东西。
First of all, did you know that the outline or the border of your lips (called the vermillion border) is a special feature of humans only? This transition line from your skin to the pinkish-red part of your lips is found only in humans—no one knows why.
The lips appear red because of the underlying blood vessels. Arteries are blood vessels that carry blood back to the heart. The arteries and veins are connected through a series of blood vessels called the capillaries.
These red-colored blood filled capillaries are close to the thin skin on your lips, so your lips appear red.