One more for the night, promise... settings.get doesn't seem to return correct values when a plugin is loading. Paste the following into a test py plugin file:
settings_base = sublime.load_settings('Preferences.sublime-settings')
print('color_scheme is:', settings_base.get('color_scheme', 'no color scheme'))
...and regardless of whether you have color_scheme set in your Preferences, this will print "color_scheme is: None".