Meaning: owe money to the bank. Before computers, all bank records were done in pen and pencil. If you owed money to the bank they would give you a bank statement that used red pencil or ink. The opposite of this idiom is in the black.
Example: Joe is in the red with the bank and they wanted their money back. He didn't see how he would ever get out of debt.See this Idiom in a story: Colour