Vermilion border
The sharp edge where the red part of the lip meets the surrounding skin.
Also: lip line
The vermilion is the red or darker part of the lips, and the vermilion border is its edge. It is often slightly raised and catches light, which is why makeup artists highlight just above it. Lip liner follows or slightly extends it. The colour of the vermilion comes from thin skin over many blood vessels, and varies with a person's skin tone. It is also the line where lip tattoos, such as the Ainu sinuye, were traditionally drawn.