I am not completely sure whether this is even supposed to work, but reading the documentation of pdfx
it is my understanding that while PDF/A by default uses a RGB profile, it is possible to specifically ask for a CMYK profile. The manual says the following (p. 12): "For users who have a specific color profile they wish to use, it is possible to do so by including a \setRGBcolorprofile or \setCMYKcolorprofile command in the .xmpdata file."
I tried the following MWE:
\RequirePackage{filecontents}
\begin{filecontents*}{\jobname.xmpdata}
\setCMYKcolorprofile{coated_FOGRA39L_argl.icc}
{Coated FOGRA39}
{FOGRA39 (ISO Coated v2 300\% (ECI))}
{http://www.argyllcms.com/}
\Title{Title}
\Author{Author\sep}
\Language{de-DE}
\Keywords{keyword1\sep keyword2\sep keyword3}
\Publisher{Publisher}
\end{filecontents*}
\documentclass[]{scrbook}
\usepackage[a-3b]{pdfx}
\begin{document}
Test
\end{document}
While this is a valid PDF/A according to Adobe Acrobat – the preflight test does not produce an error – the color profile used is IEC sRGB. And if I actually include CMYK images, it does not pass as proper PDF/A since the color profiles of the PDF and the included image differ.
pdfx
simply seems to ignore the color profile set in the .xmpdata
file.