Maybe this is silly, but...do you actually have a black ink cartridge installed?
If so, what level is it being reported at?
Is only your color getting low or are both getting low?
If you take out the color cartridge and run a test print, is it successful?
Have you checked your printer settings? You can sometimes set it to print with only color ink or only black ink or both.
It's quite possible that your color cartridge is getting low simply because your printer is using it to print black text/images. And when color is used to print black objects, it gets drained rather quickly.
It might help to know your OS and the make/model of your printer.