Today’s idioms: “to be in the black” and “to be in the red”

In the days before computers, bookkeepers wrote positive numbers in BLACK ink, and negative numbers in RED. Today’s idioms come from this practice.

So, “to be in the black” means to have a profit, or a positive bank balance. “To be in the red” means the opposite, i.e. to have a loss, or to owe money to other people.