I use a Brother printer in my office for those rare times I actually need to print something. Brother has this devious scheme to get you to purchase ink on a regular basis, the printer won't let you print, even in black only, if the color cartridges are low on ink. So, I needed to print something this morning. Black is full, magenta is empty. No time to go buy ink. I found a solution.
They must use an optical sensor to determine ink level in the cartridge, each cartridge has a clear section that would be blocked by ink if full. So, I taped over it and voila, printer thinks magenta is full and I can print in black again.
Fight the power!