If you're talking about the save file changing colour based on who is the main character, you could just reskin the difficulty select to do that. Example mockup:
You can use the CHECK_HARD condition in your events to determine whether stuff happens in Eliwood or Hector mode, and you can either leave the hard mode bonus levels intact in the Chapter Data Editor if you want Hector mode to be harder or change them to 0 if you want it to just be different but not more difficult. Additionally, there's a "make it hard mode" assembly hack floating around somewhere that you could use for a transition from one mode to another à la FE7 giving you the option to start from ch11 if you don't want to play Lyn mode. The biggest hurdle would probably be some check to change which character can Seize/use the Supply command based on whether or not it's hard mode.
All this being said, I would not recommend this to a beginner; all the conditions and checks you'd need to mess with are hardly worth cannibalizing the difficulty select. You might be able to use easy mode for a third book, but I don't think there's a CHECK_EASY event so the possibilities of what you could do with it would be very limited.