Force always Duplex printing

[Anonymous]

> > I tried this and it works on most printers here. The exception is the HP 4600dn's. It just gets ignored. I tried to google for an answer but came up empty. Is there some other catch? (I'm using the ppd as linked to by linuxprinting.org, btw.)
>
> Post (an URL to) the PPD you used. There are several different ones
> around, but
> as far as I can see, there is no reason why it should not work (except
> the printer
> has no duplexer installed, of course).

This is the one I used:
http://www.linuxprinting.org/download/PPD/HP/color_laser/hp_color_LaserJet_4600.ppd

Would a similar trick work for forcing color/grayscale, btw?