If you're looking to default the printer to B&W printing, the best place to set this is in CUPS, the Mac's printing system. I'll do my best to type this from memory. Open up a browser and in the address bar type in localhost:631/printers
If you get a screen that says web interface is disabled, copy the content within the quotes on that message, open up terminal, paste it in and hit enter. You can then refresh the page to now show a listing of all the print queues set up on the Mac. Click on the correct queue's link. On the next screen, click on the right-most drop down menu and select Set Printing Defaults. The next screen will present you with clickable categories. I'm not sure if this applies to the driver available for the Phaser 6500, but there should be one for Image Options. Somewhere in that list you will see an option for color. It may be called Xerox Black and White Conversion. If so, turn it to on. If the menu item is called something else, the value will be Monochrome or Black and White. Once you set the correct option, click Save Defaults at the bottom of the screen.
You will be prompted with a pop up to log in to authorize the changes. If you don't know your username, you can open the Apple icon in the upper left corner and it will be the last option in the list to Log or Sign out. Type the username as you see it there minus the ... and then it's the same password as you would log in or unlock the Mac. **If your user account doesn't have a password, you won't be able to save the settings. CUPS requires an admin user that has a password. If that's the case, you will need to create a temp admin user with a password to make the changes.**
Completely quit out of the application you want to print from and once you reopen it and try to print, the default should now be black and white.
