A woman’s outer genital lips are proportionately the same thickness to her facial lips. Desmond Morris describes this as “self-mimicry,” as it is intended to symbolize the female genital region. The lips can be made to appear wet either by the use of saliva or cosmetics, giving a woman the appearance of sexual invitation.
When a woman becomes sexually aroused, her lips, breasts, and genitals become larger and redder as they fill with blood. The use of lipstick is an Egyptian invention that is four thousand years old and is intended to mimic the reddened genitals of the sexually aroused female. This explains why, in experiments using photos of women wearing various lipstick colors, men consistently find the bright reds the most attractive and sensual.
The Definitive Book of Body Language
Allan and Barbara Pease 2004