This is a known error for Emacs on macOS Mojave. Apparently Emacs has been using a deprecated, and now removed, API. You can follow the thread on the “emacs-devel” mailing list.
Edit 25 JUL 2019:
Since people still seem to be getting to this question, patches for this issue were pushed the emacs-26 branch as of Fri, 28 Sep 2018. The latest version of Emacs 26 (or greater) from source or https://emacsformacosx.com/ will work. If you’re just now upgrading and you still get the error/crash with the latest Emacs, delete ~/Library/Colors/Emacs.clr and restart your new Emacs. Emacs will recreate a non-corrupted version of that file as documented here.