So, you’ve got the shiny new iOS 4.2 on your iPhone 4 and you have enabled AirPrint sharing on your Mac, using either the free Hacktivator or one of the commercial packages. You are very satisfied (albeit a bit doubtful about actually using it in the future), except for one thing: it prints in color, and you really wish it could print in grayscale, because toner is not cheap.

When printing on the Mac this is easy: in any application’s print dialog, you change the specific settings of your printer to convert to grayscale and save a new preset. Then you just select “use last selected preset” and you forget about it, limiting yourself to changing it to color whenever you feel like wasting money (or printing money, if you’re that kind of naughty boy.)

AirPrint, on the other hand, just doesn’t have any setting, aside from the number of copies. You can’t even choose which pages to print. It’s either all or nothing. And it prints in color. It all seems lost, but fear not! OS X’s print subsystem is based on — or rather, it actually is — CUPS, which stands for Common Unix Printing System. Therefore all you need to is fiddling with it to change the actual default settings. How do you do that? It’s very, very simple.

Go to http://localhost:631/ (the CUPS configuration runs via a pseudo-webserver on port 631; don’t worry, this only works on your own machine for your own machine), and click on Administration on top, then click on Manage Printers. Click on the name of the printer in question, and in the Administration pull-down menu choose Set Default Options. What you see now depends on the actual printer you have, but it should very closely resemble the options you get in the OS X print dialog. In my case, the General tab shows a setting called  Color Mode that lets me choose between Color and Grayscale. I set it to the latter, clicked the Set Default Options button, typed in my username and password, and off I went.

Note that you must have administrator rights, and you have use your short username, the one in lowercase with no spaces. It’s the same one that gives the name to your own home folder.

(Many thanks to Marco F. for the input.)

Keep in mind that this is for my own printer, and that I use OS X 10.6 “Snow Leopard.” Results may vary with different printers and/or operating systems, so make your own tests. If your printer outputs pages “face down,” this will probably work as it is. Most ink-jet printers on the other hand output prints “face up,” so some adjustments will be necessary.

In any case the steps for my own printer are very, very easy (once you’ve figured them out correctly):

1. Print all odd pages in normal order
2. If the total number of pages is an odd number, take the last sheet and put it away for the time being
3. Take the (remaining) sheets and put them back into the tray after rotating them 180°. Do not flip them in any way!
4. Print all even pages in reverse order
5. If you put the last sheet away in step 2, put it back at the end of the stack
6. There is no step 6

Enjoy.