Move the "Show Group Colors" toggle from a per-editor option to a single user preference in the Animation preferences.
The old "Show Group Colors" toggle had to be set per editor, and was on by default. This meant that disabling group colors would require an action for every file, for every editor.
The Grease Pencil interface warns about this:
(the warning is not part of the below test builds though)
Test builds
Versioning Warning
Note that Blender's sub-version (a hidden number that's used for managing versioning code between various commits of a single release) hasn't been bumped yet in this commit. This means that the code that sets the default to ON of this new user preference is run every time Blender loads the user preferences. Expect the user preference to act like it wasn't saved -- it /is/ saved, it's just that the saved value is overwritten every time it's loaded. This will be resolved when the patch is committed, as only then I can reliably bump the sub-version.

