Question: I'm looking for an OS X editor (preferably a dedicated Forth editor, but I doubt it exists) that has/can be customized to change the font color of Forth variables, programs, etc. If not, is there any sort of workaround?

Answer: 
Look no further !!

I'm currently programming in FORTH using the 'Visual Studio Code; editor.
Has (installable) syntax-highlighting for many languages, including FORTH.

Have a try:
<https://code.visualstudio.com/download>

It has even automatic indenting, AND code folding!!

...and let me know if you like it like I do.

Robbert / PA3BKL
I would suggest Gedit and if you change the forth.lang in the app you can change the syntax highlighting