It's pretty simple.
You can find all the factions' color combinations in:
...\assets\DLC\Expansion1\Gameplay\XML\Interface\CivBEColors.xml
The XML table is "Colors".
The percent is based off of 255. So, that will give you your RGB numbers. That's how I created the "globes" diagram of all the factions colors -- they're exact. It gives a good overview while designing so you can walk the fine lines between each faction's hue. I originally wanted more orange mixed in with Chungsu's color, but when putting them alongside the African Union, they started to blend together more than I wanted.
With a little googling, you can find a handy website that can do the work for you.
I used this one (
thank you, Hew) to quickly generate the decimal numbers, because I tested a ton of different colors to see what they would look like in the game. I mainly worked off of the hex color labels between that and Photoshop, and copied the decimals into the mod.
The existing icons can be found in:
...\Resource\dx11\Expansion1UITextures.fpk
You can unpack the DDS files with Dragon UnPACKer. The six files you're looking for when modifying an existing leader start with "civsymbolscolorxp1_" and end with the icon size number. You then have to unscramble the DDS files you've extracted with the amazing, handy-dandy
DDS unpacker.
Note, if you're making your own atlas, just make sure you either match the index number with the existing one, or copy all the other atlas symbol types: "gradient", "glow", and "atlas" (which is the alpha version). You'll have to do that latter step anyway if you're not only changing the colors, but the shape of the image itself. (I'm not sure if the game really even uses the "gradient" and "glow" atlases, since the existing Chungsu ones aren't even the correct image -- the wave has a different shape to it. Post #10 above shows these inaccurate images.)
Use Photoshop or any similar application to modify the PNG file you've created and then save it as a DDS file again; I use 8.8.8.8 ARGB.
Plop your atlas files into the mod and make sure to import them into the VFS by setting it to "true".
If you kept the names the same and are overriding the original ones, that's it. Otherwise, you'll have to enter your new atlases into the XML under the IconTextureAtlases table. Then, in the Civilizations table, enter the XML names of the atlases, and change the index location if needed, so the faction utilizes them.
I think that's about it. Above, I said it was simple and it seems like I wound up writing a whole lot, haha. But, it really is, once you try it.