The graphics on the left show the desired or Ideal Pigments accompanied by representations of fairly common values of Subtractive Colour errors in our best pigments.

In the industry we evaluate the results with densitometry. Note that each Primary Subtractive Colour contains a significant amount of the other two primary colours.

• Cyan contains a large amount of Magenta and Yellow
• Magenta contains a large amount of Yellow and to a lesser degree Cyan
• Yellow contains Magenta and Cyan

This has always created quite a problem for Prepress Technicians. Now the writers of Graphic Arts computer programs have to tackle the problem too. It is a straight forward situation for scanned images created by sophisticated high end and desktop scanners, for all colour correction can be applied before the bitmap is created (if we make sure that the images are not further corrected at output). However when colouring objects in Pagelayout and Drawing programs we must use some kind of colour reference system and mostly ignore how the colours appear on the Monitor. The Choosing correct DTP Colours page describes the habits we should form to guarantee best output.

Results are far from perfect and extensive measures have to be taken to balance out as much of the errors as possible. How this is done might be best described by a look at the old traditional separation method of Double Overlay Masking where, as an example, colours that print with high amounts of yellow and some magenta will have to be reduced in the yellow plate because the magenta also contains yellow.

View the Double Overlay Masking page.