This proverb suggests that beauty is unimportant. Beneath the outward appearance people are all much alike. As Plutarch wrote: When the candles are out all women are fair.
The proverb is thought to have originated in the 17th century. It is often attributed to the English poet John Gay, who wrote in his poem Trivia (1716):
In a dark room all cats are grey,
For love sees not varieties.