Comment 22 for bug 574717

My solution is similar, I then checked the result in Scribus 1.5 where the print preview lets you toggle the channels. But you could also check in the list of colors (e.g. in the dialog to replace colors) that all are CMYK.

Here the pdf2cmyk script with rich black and the ISO coated profile, I also prefered PDF 1.4.

#!/bin/bash
gs -o ${1%%.pdf}_cmyk.pdf \
       -dNOPAUSE -dBATCH -dSAFER \
       -sDEVICE=pdfwrite \
       -dPDFSETTINGS=/prepress \
       -dEncodeColorImages=false \
       -dEncodeGrayImages=false \
       -dEncodeMonoImages=false \
       -dOverrideICC=true \
       -dHaveTransparency=false \
       -dCompatibilityLevel=1.4 \
       -dRenderIntent=3 \
       -dDeviceGrayToK=true \
       -sDefaultCMYKProfile=/usr/share/color/icc/ISOcoated_v2_eci.icc \
       -sOutputICCProfile=/usr/share/color/icc/ISOcoated_v2_eci.icc \
       -sProcessColorModel=DeviceCMYK \
       -sColorConversionStrategy=CMYK \
       -sColorConversionStrategyForImages=CMYK \
       $1