The astute will notice that there are two green wagons in WU. One is the Freedom wagon, which is supposed to be green, and the other is the Crusaders wagon, which is supposed to be white, but isn't.
The Crusaders textures were disabled in WU years ago due to a copyright. They still exist, you just need to re-enable them. Here's how.
Close any running copies of Wurm Unlimited.
If your archive manager supports JAR files skip this step. Otherwise, download and install 7zip.
The # character is used at the beginning of a line in the config text files as a way to comment it out/disable it from being read in as a setting. When you remove the leading # from a line, it enables it and WU will read it as part of its settings. What we're about to do is re-enable lines that were commented out to disable the Crusader textures.
Navigate to your WurmLauncher\packs folder.
You should now be able to see the Crusaders textures for wagon, tabard and banner.
NOTE: If you get “file in use” errors when updating the files, it is because a copy of Wurm Unlimited is stuck running in the background. Use Task Manager to close all copies of Wurm Client, or reboot to fix the issue.
I was able to do it in Linux Mint. Use your built-in archive manager (FileRoller is what is in Linux Mint) in place of 7zip. The procedure is pretty much the same. Right click the .jar file and Open With → Archive manager. Edit and save the .txt file and a popup will ask you if you want to update the .jar file. Update the .jar file. Do this as per above and you're set.