It’s verified on each of my cubase projects.
If Stereo In or Stereo Out Bus are not present in the project window, but only in the MixConsole,
it’s the case when you’re starting with an empty project,
you cannot change their track color.
You have to made them present in the project window, by exemple clicking Write automation button, and deactivate it after.
So a track is created in project window, and you can change his color, in the project window and/or in the Mixconsole.