I work a lot with brand colours specified as Pantones, but often printed as CMYK. Naturally, I want those conversions to be as accurate and as consistent as possible, and I keep my Adobe applications’ colour settings synchronised (via Bridge) to use the ‘Europe General Purpose’ preset.
What I've checked:
Testing shows me that as long as the colour settings and document profiles all match between applications, Adobe InDesign and Adobe Illustrator will provide identical colour values when converting a Pantone swatch (chosen from the Pantone+ Solid Coated book, FWIW) to either RGB or CMYK values via the applications colour palette.
Pasting that derived RGB (or hex) value from either of those apps into Photoshop's colour picker also provides CMYK values that match the derived values from InDesign and Illustrator (though there is some rounding off of the numbers -- InDesign and Illustrator both provided converted CMYK values to 2 decimal places, whereas Photoshop deals in integers).
Everything seems to agree. That's good, right?
Creating a test document
Now I create a document in InDesign (giving it a colour profile to match the working space) and in it create two squares of colour, one using a Pantone swatch and one using the converted CMYK equivalent of that same Pantone colour reference. I then export that document to PDF using InDesign’s native export (as opposed to using Distiller, which, if I'm honest, I never have time for in my workflow).
My export settings are very close to PDF/X-1a standard, the only real different being that compatibility is set to Acrobat 5 rather than 4. Compression and Printer mMarks are irrelevant for this test, and the Output settings are ‘Colour Conversion: Convert to Destination (Preserve Numbers)’ and ‘Destination: Working CMYK - Coated FOGRA39’, with profiles not included.
Crucially for this test, all spots are set to convert to process in the Ink Manager.
Since the same colour profiles and synchronised Colour Settings are being used across the board, I would expect the colour values derived from 'Spots to Process' during InDesign’s export to PDF process to exactly match the colour values derived by converting the same Pantone colour to CMYK within InDesign itself.
Yet when I open the exported PDF in Acrobat and check the values using Print Production > Output Preview > Separations there is a discrepancy between the CMYK values of the colour converted during export and those of the colour converted via InDesign's colour palette at layout stage. It's not a big difference — a percentage point here or there — but it’s not zero.
I also note that the CMYK values for the unconverted Pantone swatch provided by InDesign’s Separations Preview palette (which show that ink as CMYK values because Ink Manager is telling it that all spots will be converted to process) match the ‘converted at export’ values seen in Acrobat, not the ‘converted within the colour palette prior to export’ values.
So what’s going on here? Are there two different conversion algorithms in play?