You should have a file /etc/cups/ppd/xxxx.ppd, where xxxx is the name you have configured your printer with. Please post (an URL to) this PPD.
*OpenUI *ColorModel/Output Mode: PickOne
*OrderDependency: 10 AnySetup *ColorModel
*ColorModel CMYK/Color: "<</cupsColorOrder 1/cupsColorSpace
*ColorModel Gray/Grayscale: "<</cupsColorOrder 0/cupsColorSpace
Thus, if your printer is configured properly, you simply could (either by using the lpadmin command or the "configure printer" from the web admin) set the default color model to Grayscale. You could even edit your printer's PPD "by hand" by replacing
And you may easily set up two print queues - one configured for color printing and the other for b/w printing as indicated above - pointing to the same physical printer.