I have a color printer but would sometimes prefer to print grayscale (for documents where color and quality does not matter)
As far as I can see there are no such settings in the preview printing menu, what is the right approach here?
It depends on each printer. Looking at your picture, looks like the color option is under "Printer Features".
Once you find good settings, you can create presets using the second popup menu from top to bottom (on the image). Then you can choose between your preferred settings easily.