I printed a grey for the lift hazard markings on my HMS QE model but used full colour mode and think that came out OK - of course if you look at high enough magnification the individual CMYK dots can be seen but they aren't visible to the naked eye. I think if you have a printer that can produce a decent colour photo print, it should be OK.

The problem I couldn't overcome was getting an exact colour match with the grey paint used for the rest of the deck. Maybe if you are good with custom colour profiles and have a colour measuring device for calibration, that would be possible but it was beyond me.