Always prints in colour (LaserJet Enterprise 500 color Printer M551n)

In the past few days the printer has decided to always print in colour, even when I specify "Black & White" in the "Color options" menu. ...and yes, even if this is the one and only change that I make before telling it to print, and then I simply "OK" and "OK" in order to send the document to the printer.

Prior to this (ie: for the first few days of ownership) the Black & White option did force it to print in, well, black and white. How

I have made no software changes. Why is it now misbehaving and, more importantly, how do I correct it?