LAB Spot Colours in Adobe Software
What is a spot colour?
A spot colour is a special premixed ink that is used instead of, or in addition to, process inks. They require their own printing plate on a printing press. Use spot colours when few colours are specified and colour accuracy is critical. Spot colour inks can accurately reproduce colours that are outside the gamut of process colours (CMYK). However, the appearance of the printed spot colour is determined by the ink, not by colour values you specify.
When you specify spot colour values, you’re describing the simulated appearance of the colour for your monitor. This is where the issues start. The earlier versions of Adobe software used CMYK values to simulate the appearance of spot colours. This didn’t give an accurate on-screen representation of how the spot colour would print as a spot colour. But it did give a fairly accurate representation of how the spot colour would print if converted to CMYK.
Spot colours in the latest versions of Adobe Software
In previous versions of Adobe software, for example Illustrator, Indesign and Photoshop, the spot colour libraries supplied for Pantone used CMYK colour values to create the preview of the spot colours. This meant that when spot colours were used, but weren’t required for printing, they would be converted to the CMYK values provided by Adobe.
In the latest versions of Adobe software, the Pantone colour libraries use LAB values to display spot colours. This gives a more vibrant and accurate colour representation (on-screen) of how the spot colour will look when printed as a spot colour. BUT, if the LAB spot colours are to be printed in CMYK, this is where the problems occur. When converting the newer LAB spot colours to CMYK, the CMYK colour values differ to that of the old CMYK Pantone colours and also ‘dulls’ the appearance due to the colour gamut limitations of CMYK.
How to fix this Issue
If you use spot colours and they will be printed as spot colours there won’t be any issues. The on-screen LAB preview of the spot colours are a more accurate representation of how the spot colours will print.
If you use spot colours, but they will eventually be converted to CMYK for printing, it would be safer to use the older CMYK Spot colour libraries which can be imported into the newer Adobe programs and the on-screen previews will give a much better representation of how they will look when converted to CMYK.
Alternatively, you could just convert the spot colours used in your swatch panel to CMYK colours before exporting to PDF. This will give you a fairly accurate representation of how they will look when printed in CMYK.
How to import old CMYK colour libraries
There is a very good article on the Adobe website that explains how to import older CMYK Pantone colour libraries: