Not sure what printer you have, but you should be able to change the print settings.
On the print page (after you select print from the application and before selecting the print button on the preview/settings page), select the drop down box (it should be defaulted to the application, i.e. if printing from Safari it will say Safari in it), within that box select Printer Features and a new drop down box should appear that will allow you to select color or grayscale. This would allow you to set it for that print. I couldn't tell you at this time how to make that setting stick as the default though. If you have the correct print driver installed this should work for you.