How to convert RGB to CMYK while keeping "pure" magenta and black


I’m writing a tool to automatically generate a large number of PDFs, and one of the libraries I’m using can only output PDFs in RGB format. I’m now using Ghostscript to convert the RGB pdfs into CMYK pdfs. This is in preparation for sending them to a professional printer to have them printed into a book. The PDFs have only 2 colors – black (including some greyscale) and magenta. The printer requires that the colors be isolated to the 2 color plates magenta and black. The problem I’m running into is that my current ICC profiles used with Ghostscript is leading to 99% magenta and 1% yellow for the magenta areas, and very mixed (60-70% coverage each for CMY and K) for the black and grays. The goal is 100% magenta for the magentas, and 1-100% k(black) for the black and gray areas.

It’s my understanding that I can correct this with the proper ICC profile, but I’m a little bit out of my depth with this color theory. I am a software engineer though, so I expect that if someone can point me in the right direction I should be able to take it from there. Does Chromix offer a tool that will help me to solve this problem? Am I looking in the right direction when thinking I can solve this with properly created ICC profiles? I’m running macOS, but could get a virtual machine up if a tool that solves this issue only exists on Win or Linux.

tldr: How to convert RGB pdfs into CMYK pdfs with ONLY the M and K channels of the CMYK output being used.

Thanks for any insights!


Your best be would probably be a device link profile set to preserve black and preserve magenta. You would want to have the blacks and grays in RGB to have equal amounts of RGB (127,127,127 for a mid tone gray) and magenta be 255,0,255 RGB to convert to pure magenta. It still might need some tweaking in something like Pitstop after the fact to make it 2 color.