The excerpted PDF on writing the code is here:
I ran a simple test and it works for me. The .mrk file goes in /Library/Application Support/Adobe/PrintSpt:
The bad news is looking at the API there's no command for changing the color of crops or bleeds. Looks like you can change dimensions and position but not color. Maybe Olav Kvern would have some info over in scripting.
I had the wrong directory all along. I was going to:
The marks now appear fine.
There's hardly any information on the topic though, and I don't think anyone has even asked the question "what is there besides default trims" before.
when re-visiting the situation to see if it would help other posters trying to make their crop-marks black only,
The David Blatner PDF I linked to covers the cropmark coding, but it really doesn't look like mark color is an option.
Here is a script I've been thinking about trying for awhile:
It clones the active document, places the pages of the original on the pages of the clone, makes a slug with crop marks and exports. You choose a PDF preset and color for the cropmarks. The PDF gets saved in the same directory with the ID file. The globals at the top of the script let you adjust length, offset, and weight
Seems to work well, haven't tested on a complex doc yet.