Most printers use a combination of black toner/ink and color toner to print documents. The reason is it aids in a number of things, like:
- Smoothing of fonts
- Recreating more accurate greyscale tones
- Providing more tone/depth to the actual color black
Depending on your printer, you may have the option to force it to use only the black ink/toner cartridge for greyscale printing, but you may notice a loss of quality or smoothness in fonts. For instance, Brother calls this setting "Mono".
Another option that might be available is if your printer allows to print when a cartridge is empty or missing. If your printer supports it, you can simply remove the color cartridges and print only using the black one.
Most printer manuals will include a disclaimer that printing in black and white does not mean that only the black cartridge will be used.