Nargott
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There's an improved option: don't ask the player about anything at all (no need to create unnecessary dialogues).
Activate the change of the base color when two conditions are met simultaneously: placing a fort and having the hero in the city garrison (the color changes to match the hero's class). If the hero is not in the garrison during the placement of the fort, no change should occur.
In other words, for regular players, it's as if there's no color change at all (no dialogues), but if the player wants, they can activate it (by placing the hero in the garrison at the moment the fort is built).