ColorSync Utility
You want to use ColorSync Utility. Launch it, then find your device in the list of printers. Change use the drop down labeled Current Profile to choose other, then navigate to /System/Library/ColorSync/Profiles.
Select one of the gray profiles and click the Open button. Try printing. You may have try a few different profiles until you find the best one. Then just close ColorSync Utility. The grayscale profile is now permanently assigned to your printer. ColorSync Utility does need to remain running.
Good luck.
Alternatively, you might try setting the default option for the printer in CUPS instead of changing the color profile. To do that, open Safari and go to the following URL:
http://127.0.0.1:631
You will get to the CUPS home page. CUPS is the printing subsystem in OS X and most other Unix systems. Now click on the Administration tab of the page and click on the Manage printers button.
Somewhere in this process you will be prompted for a username and password. Use your short name (the name of your home directory) and your password.
Now click on the link for your printer. You will come to a page for that printer with two drop downs at the top: Maintenance and Administration. Click on the Administration drop down and choose set default options. You will get one or more links at the top of the page followed by some options you can set. Find the color settings and pick grayscale, then click the button at the bottom of the page called "Set Default Options"
Now you have changed the default permanently. While you are in that section you might also set other defaults you don't like.
Hope that helps.
