unfortunately, the Options interface isn't flexible enough for that. it only has simple inputs like checkboxes and dropdown menus, nothing i could use to show colors or themes.
sorry about that! i never rely on just hue to convey information, so there should be other visual cues you can look for. the red is much lighter than the brown (about double), and the foreground color is different too: white on dark brown for player control, amber on red for enemy control. if that still doesn't help, the text shows exactly which civilization controls the district.
i've also considered using player colors instead of red, yellow, and brown, but i would still need a clear way to highlight enemy units and territory, and i haven't found a solution that i like yet. i could also use my other alert style (amber & black) to highlight enemy-controlled districts. i'll play around with it later and see what sticks.