I think when you select an army, even in the terrain map mode the game should color baronies if they can supply the army or not like this:
Imperator: Rome have a supply mapmode which work something like what I mentioned above but I don't like to have to swap between several map modes, like one to see terrain and one to see supply.
- Green, army is well below the supply limit
- Yellow, army is close to supply limit but not over it
- Red, army is over the supply limit.
Imperator: Rome have a supply mapmode which work something like what I mentioned above but I don't like to have to swap between several map modes, like one to see terrain and one to see supply.