sdaau, the command you used for trying to convert your PDF to CMYK was not correct. Try this one instead:
gs \
-o test-cmyk.pdf \
-sDEVICE=pdfwrite \
-sProcessColorModel=DeviceCMYK \
-sColorConversionStrategy=CMYK \
-sColorConversionStrategyForImages=CMYK \
test.pdf
Update
If color conversion does not work as desired and if you see a message like “Unable to convert color space to Gray, reverting strategy to LeaveColorUnchanged” then…
- your Ghostscript probably is a newer release from the 9.x version series, and
- your source PDF likely uses an embedded ICC color profile
In this case add -dOverrideICC to the command line and see if it changes the result as desired.
Update 2
To avoid JPEG artifacts appearing in the images (where there were none before), add:
-dEncodeColorImages=false
into the command line.
(This is true for almost all GS PDF->PDF processing, not just for this case. Because GS by default creates a completely new file with newly constructed objects and a new file structure when asked to produce PDF output — it doesn’t simply re-use the previous objects, as a more “dumb” PDF processor like pdftk does {pdftk has other advantages though, don’t misunderstand my statement!}. GS applies JPEG compression by default — look at the current Ps2pdf documentation and search for “ColorImageFilter” to learn about more details…)