The machine will print using color toner if the print job was being sent in color mode. Even if the document looks black and white, the software program may tell the machine to print using color.

Even when the printer driver is set to Monochrome mode. The machine will still use the color cartridges during initialization of the machine when the machine is turned off/on, during the cleaning cycle, and during the calibration and registration process, which results in the toner cartridge rotations. Due to that reason you will eventually get a 'Replace Toner' message for the color cartridges.