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I've tried some scripts and various tricks I found online, but I'm starting to be near my wit's end.

If I do identify -verbose mwe.pdf | grep Colorspace, I get Colorspace: sRGB. I need the CMYK for a print shop.

What to do?

\documentclass{article}
\usepackage{xcolor}
\definecolor{textcolour}{cmyk}{0.6042,0.6875,0.0000,0.8118} % dark
\definecolor{bgyellow}{cmyk}{0, 0, 0.10, 0} % light yellow 

\begin{document}
\pagecolor{bgyellow}
\textcolor{textcolour}{Foo}
\end{document}
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    Try \usepackage[cmyk]{xcolor} – Ulrike Fischer Sep 4 '19 at 11:43
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    Imagemagick's identify does not give reliable output. It will return RGB colour space regardless of what is in the PDF. To really see what is happening you need to use Acrobat's (or some other applications's) preflight tools and check for non-CMYK colours. Other options are to use GhostScript to produce separations and check to make sure things separate as you expect. Or import the PDF into Scribus and see if any imported colours are not in CMYK. – David Purton Sep 4 '19 at 11:57
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    This bug report (github.com/ImageMagick/ImageMagick/issues/431) adequately explains why ImageMagick is not the right tool to determine what colour spaces a PDF uses. GhostScript, as best I can tell, can not do this either. – David Purton Sep 4 '19 at 12:08
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CMYK is not a standard. Print shops will not (only) require CMYK, but a real standard document like PDF/X-4. The most important requirement for that standard is colour management. See this example to get an idea what needs to be done.

In easier words: By choosing a colour profile you define how values like 0.6042,0.6875,0.0000,0.8118 are converted to physical colour. This ensures that

  1. your document is printed the same way in all (good) print shops.
  2. The colours match against an absolute standard. I.e. the university logo looks the same as on other coloured things (documents, walls, labels) created by different tools and printing processes.

Using PDF/X-3 or -4 you can embed RGB images. The print shop will convert the colour values for their machine using the profile you supplied in the PDF.

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