Gareth -- I was able to solve the problem by switching from the recommended "pxljr" driver to the alternate "hpijs" driver. Go to System -> Administration -> Printing -> HP ColorLaserjet 3600, then click to change the make and model, choose HP -> Color LaserJet 3600, click on the "hpijs" driver, and accept the new postscript-printer-definition (PPD) file as is. That should solve the problem.
Gareth -- I was able to solve the problem by switching from the recommended "pxljr" driver to the alternate "hpijs" driver. Go to System -> Administration -> Printing -> HP ColorLaserjet 3600, then click to change the make and model, choose HP -> Color LaserJet 3600, click on the "hpijs" driver, and accept the new postscript- printer- definition (PPD) file as is. That should solve the problem.