If that can help 2 link of my research on this subject.

[JeremieRousseau]

I not know if that can help you but I tried to show a PPM P6 picture on ACeP 32000 colors
an at a moment I call https://www.beck-elektronik.de
A German company who make controller for Eink.

And I made with a NodeJS a little program to transform my imageData who store RGBA from my Javascript to my server with that link
https://en.wikipedia.org/wiki/Netpbm
to generate a PPM P6 file but i not searched to transform RGB in CYMK

My project :
https://essaie.fr
https://essaie.fr/download-animation-software