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.
This answer was provide by Dr. Beth Ann Ditkoff. This and other interesting answers can be found in Ditkoff's book, "Why Don’t Your Eyelashes Grow?: Curious Questions Kids Ask About the Human Body." Republished here with permission.