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: 
[UltraEdit](http://www.ultraedit.com/products/uex.html) has a Forth syntax highlight mode. MicroProcessor Engineering have an up to date copy in there [downloads](http://www.mpeforth.com/arena.htm) page.
I would suggest Gedit and if you change the forth.lang in the app you can change the syntax highlighting