It would be really great if this were undoable with cmd-Z.
Interestingly, changing the color seems to mark the document as dirty (if I understand the meaning of that), and when I quit OF after making only a color change, it asks if I want to save. But if I choose
don't save, the color change is
saved anyway.
I was able to undo it by downloading a plist editor and following the instructions in
this post of Tim's.