You don't have to buy a new printer. What Roland was talking about was not buying a new printer, but telling the computer that you have a different printer that only prints in black and white (even though you're actually pointing the computer to the same printer that you're already using). But you don't even need to do this (if you have a Mac, anyway; I don't know about Windows).
On a Mac, when you tell it to print (from any application), you get a dialog box with options for your printer, which are set by the printer driver. Somewhere in there, usually under something like 'color' or 'quality', you'll have the option to set it to black and white. You can then save that option as a preset, which will be available as an option whenever you print (from any application) in future. If you save another preset for colour output, you can switch back and forth between them as needed when you print.
I know these options are also available on Windows; what I don't know is whether you can save them as presets.