Same as how locomotives can auto-color themselves based on the train stop, could there be a setting to auto-color wagons based on the locomotive color?

Good idea,
I guess if you don't tick in the locomotive that you want to color based on destination, all wagons can just inherit color from the locomotive and that's it.
I'll implement this to the next version when I have the time.
Thanks!

(Could you maybe write in this thread when it's implemented? I use a ton of mods and don't check them all nor do "update all", heh)

Feature added in 2.1.0 version!

Now if you just color the locomotive manually, all wagons will inherit the same color.