What causes nipples to darken?

Nipple/ areola: During pregnancy the areola become darker w thicker tissues & increases in size. The Montgomery glands on the areola become larger. For some women, the areola returns to their pre-pregnancy size – but not for all. Although the color of areola may lighten after pregnancy, it is unlikely to to return to pre-pregnancy coloration. http://pregnancy.lovetoknow.com/wiki/Changes_in_Areola_When_Pregnant