How can I fix a missing color in emacs?
After banging my head against the wall for a while, I found http://permalink.gmane.org/gmane.emacs.macintosh.osx/6639


  Delete this file:
  
  ~/Library/Colors/Emacs.clr
  
  and dark cyan should come back.


So I quit emacs, moved that file to ~/Library/Colors/Emacs.clr-, started emacs again... and it worked. I hope this fix is useful to someone else.