Metamerism occurs when the two colours appear to be identical under one light source but different under another light source. This phenomenon is mainly caused by the variations in pigments, dyestuff or materials.

Example: Digital prints were done on a two digital printers using same ink manufacturers but one was using a CMY to make grey and the other just the black. Test files were sent to various mills, and the resulting prints looked good in a light box under D65 light, but bad in 'shop' lighting, colours now looked different to their 'standard'.

Question

When digitally printing, can you control how many inks are used to make colours?

Answer

Yes. This is controlled in the Ink Settings / linearisation, you normally use a triangle of colours, but you can use CMY inks in the grey ramp, so in these cases you are using 4 or more inks.

The disadvantage of using CMY to make up the grey is that you will get more metamerism issues. For example: If you're using CMY to make grey, you measure a dark blue colour, print it and the match under D65 is good, but under other lighting there is a very obvious colour shift, this is due to the metameric distortion.  If you were to only use the black ink to make grey this effect will be reduced.

However the advantage of using CMY to make grey is that you can drastically reduce the visibility of dither in the prints, so if you only use the black ink the dither will start to be more obvious.

So when you make your linearization this is something you may need to consider. When mixing colours for screen printing it is common to use the one with less inks as it would have less metameric issues. With digital printing the mills will probably use CMY instead of black in the grey tones to get a nicer dither. You may also put CMY inks under the black to get a deeper black, or to make the black more neutral, but this will cause metamerism issues, meaning the prints will only match well in D65, whereas, if you use a grey ink you will reduce the problem.  You could also just use the black ink to create the grey ramp but you will have worse dither in the prints.