In the red
From the practise of using red ink to denote debt or losses on financial balance sheets. Likewise, in the black for businesses that are financially solvent.
Why red ink? No one really knows.
This phrase conjures up images of inky-fingered clerks in Dickensian offices scratching in ledgers with quill pens. In fact, the term is much more recent than that. The first known citation of it is in 1907 in Montgomery Rollins' exhaustingly titled Money and investments: a reference book for the use of those desiring information in the handling of money or the investment thereof:
Formerly it was customary, and is now with some bookkeepers, to make an entry of a loss in red ink, from whence arose the term ‘in the red’, always indicating a loss.