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: 
I know it's an old question, but: atom with the forth-language package. See <http://atom.io/> 

Longer answer: 

I have tried both emacs and atom on Mac OS X and both are more than adequate for editing Forth. Atom is lighter and easier to learn while maintaining many emacs-ish capabilities (cmd-shift-p in place of meta-x).

I don't have automatic indenting working in atom yet, the syntax package I found needs a little work, but this hasn't been a real problem as yet.

atom also plays well with git.

I'm using the following packages: emacs-plus, language-forth, clipboard-plus, disable-arrow-keys. The key and clipboard are for a more emacs like experience. There are more themes and color schemes than I need available, I'm using the 4-color-dark and minimal-syntax themes.
I would suggest Gedit and if you change the forth.lang in the app you can change the syntax highlighting