Literally: All cats are grey in the dark.
Meaning: At nigh everything look alike, it is hard to distinguish colors or shapes. It is used when you are in a confusing situation, usually apply to people.
Origins: As soon as the 15th century, we can find traces of this proverb. Latin borrowed to Greeks a similar expression: “Lucerna sublata nihil discrimen inter mulieres” / la lumière enlevée, il n’y a plus de différence entre les femmes/ once the lights are out,there is no way to tell the differences between women.