all cats are gray in the dark
In the dark of night, appearances do not matter (because it is so difficult to see anything). A: "I can't believe you're going on a date with someone you've never met before! What if you don't think he's attractive?" B: "Ah, all cats are gray in the dark, so it will be fine."
all cats are grey in the dark
the qualities that distinguish people from one another are obscured in some circumstances, and if they can't be perceived they don't matter. proverb The US version of this proverb is at night all cats are gray .