Playing around some with textures, it occured to me that it really wouldn't be terribly difficult to get a white-walled Minas Tirith, by changing their faction's specific settings for fortress level castles. After that turned out fairly well, I tried playing around with the opposite: Darker walls, and buildings too, for those smelly Moria orcs. What I have here ought to be compatible with any mod that doesn't change building textures, although obviously I think it would go particularly well with TATW. Won't require a campaign restart either.
The only drawback is that sometimes a settlement a notch above or below will use a single or few tiles from the other's files. That means you might have a regularly colored castle using one or two squares of white walls set for the fortress, which would look odd. And the way I've done it means any settlement of the same nation will use the colors here, so it's not specific to, say, Minas Tirith, Moria, etc. Gondor's fortresses or citadels will be white, OotMM's huge cities dark.
At some point in the near future I will probably go through all the different levels for each faction and apply the same sequence of filters to have them all following a scheme. If someone else out there can enjoy it too, all the better. I'm too busy right now, but I'd like to change some of the textures entirely... we'll see. By that time, I'm sure the mod team for TATW will have fully custom settlements out, this is just to tide me over, really.
Here are some pictures; yes, I waited a turn or two each to get some favorable weather to show off the effects.
Moria (at huge)...
Minas Tirith (Hm, either at Fortress or Citadel, I forget)...
Files are in usual structure, starting at /data/ level. Paste into the appropriate mod folder. Nothing should have to be overwritten, if you have no submods with custom settlements.