Not sure if you can edit, but you can add more colors by adding a new 'block' and editing the hex code for color to any color you want in the file called colors.txt in the same general path mentioned below in the core game files.

For the white to work, change #ffffff to #fefefe in:

YourSteamFolder\steamapps\workshop\content\282590\417744157\data\empires\colors.txt

It should be under //White near the bottom

The white one seems to be bugged. When I choose it, it comes out as the default light blue color instead once the game starts.

Thanks for this :)

Always thought there weren't enough faction colors to go around.