1. The printer have no more "color-ink" left (cyan, magenta, yellow), and the printer settings allows it to print in black/white instead.
2. If you print on a public-PC, and fx. use Chrome, you might have selected the "Black and White" setting, before continuing to select the printer. See image as example.
If you choose this setting, your print will come out in black/white, even though you choose to print in color-mode in Princh.