BEFORE STARTING - ensure your game is fully closed, backup your save games and backup any files you modify!
To backup your save-games copy and paste the Documents\Euro Truck Simulator 2\profiles
folder to somewhere safe.
This method is a more ideal than this guide since that guide suggests to copy/paste sections which could be affected by a lot of other mods, rather than the navigation colours themselves. In some cases it can cause save-games to completely fail to load (my experience). That's why I wrote this guide.
Tested on Euro Truck Simulator 2 v1.44.x. Should apply to any version and American Truck Simulator too.