Why do Japanese women paint their faces white?

According to Japanese Coolture, Japanese women painting their faces white is a reference to the geisha culture during the Heian era of Japan, which lasted from 794 to 1185 A.D. The white makeup originated in China during this time.

During the Heian era, Japan was under a strong cultural influence from China. It was during this time that Japanese courtesans, known as geisha, began painting their faces white. Rice powder was mixed with water to form the white paste used as the makeup foundation. Women wore the makeup primarily during their first three years as a geisha; in subsequent years, they relied more on their geisha arts to make themselves appealing.