The penplotter controller communicates with my modified version of Marlin and Repetier. There are a couple of custom gcodes and the polar coordinate support needed for the polargraph machine. However the gcode can be saved and used on any machine that supports gcode.

That's correct. But do not use my version of Marlin use the version that works with your corexy machine as I override the steps per mm. The Diamond treatment will not work as that uses custom gcode. Also be careful of the position of you file on the pen plotter page. The X is normally about 450 mm and the Y is 250 mm which may be larger than your machine. You can drag the plot with the right mouse button up to the top left corner of the page which is 0,0. Export and check the gcode with an editor or by loading it into something like cura or repetier before running it on your machine or at least be near the power switch on your first run.

I had another user that did a similar thing. They used photoshop to do the color separation then ran each separation through the controller and manually changed the pen between each run, the result was quite good. As the runs take so long there was not really a need to automate the two pen changes. But feel free to try. You can add a new servo command to control the pen changing servo with out having to modify the marlin code just add a new command in the comm.pde file see how the pen up and down are implemented.

Have a look at https://www.thingiverse.com/thing:2887182
Still a work in progress. Dealing with the Hatch Code so each line goes in one of the CMYK colors. I'm calculating the color sep before calling Hatch and trying to give the correct color image to each part of the Hatch routine.

When you finish the Hatch process, you turn all the paths to gcode. Is that correct? I'll have to cut the gcode generation in 4 pieces so as to embed the color changing routine (similar to up servo and down servo) in the middle and send to gcode the correct set of paths depending on the color processed.