Other forms: red inks
If you are in debt or your bank account shows a negative amount, you could say that you're drowning in red ink. Red ink is a financial deficit.
The amount of money that a person or business owes, or overspends, is red ink. If a restaurant's number of customers declines at the same time that its expenses increase, its owner will end up overcome by red ink. You could also describe this situation as being in the red. Both expressions come from the accounting tradition of writing debits or expenses in red ink, and income in black ink.