all cats are gray in the dark

dark People have no distinguishing features, and their appearance becomes unimportant, in the dark; sometimes used with reference to
a person’s choice of sexual partner: “You forgot that all cats are grey in the dark and so are uniformed policemen” (Jonathan Ross, Dark Blue and Dangerous, 1981). The proverb was first recorded in this form in 1596. It was used with sexual con- notations in a letter written by Benjamin Franklin in 1745: “And as in the dark all Cats are grey, the Pleasure of corporal Enjoyment with an old Woman is at least equal, and frequently superior.”
Variants of this proverb: all cats are black at night; at night all cats are gray.