I have an HP color LaserJet CM1312nfi. It always seems to use color toner even if I'm printing black and white text. I looked through the settings but I couldn't seem to find anything relevant.
All printers with colour ink or toner will use colour to do greyscaling around letter as an antialiasing effect. This improves the quality of the print.
The User Manual for the printer has an entry for this in its section on page 88 (User Manual Link), the setting is called "Neutral Grays", and you want the option to be "Black Only" instead of 4-color.