Convert RGB to CMYK

How to change a RGB PNG to a CMYK PNG or TIF

Install Imagick:
sudo apt install imagemagick

I believe you need the RGB colour profile and the CMYK colour profile.

Then run convert:
convert rgbimage.png -profile /usr/share/color/icc/sRGB.icc -colorspace cmyk -profile /path/to/cmyk/profile.icc -compress LZW output.tif

You can then verify the change with:
identify -verbose output.tif

For more information on colour profiles: