This stuff was originally written in 2011 for Kindle only.
Color wasn't just unnecessary, but an unnecessary strain on resources.
Anyway, it shouldn't be super complicated. But I just looked at the code and I don't quite see where the color info is lost. Setting all the mupdf.color stuff to true results in a garbled part of the image, setting color to true in k2pdfopt makes it a nice equal black but still lost. Probably some BB conversion somewhere but boy is the call chain complicated.
On Android you can easily use
https://gitlab.com/axet/android-k2pdfopt much like on PC. Most Android devices are also quite fast. KOReader's secret sauce here is doing only one page at a time instead of the whole document. It also has some definite disadvantages but otherwise the reflowing process might take quite a while on your average ereader — especially a 2011 Kindle.