Apparently I was quite wrong when I said, "once loaded, the shoulders are fixed." I just popped into the game and managed to grab a video of them changing color while doing a POV drive.

https://youtu.be/vxHFfUbYWkw

I'm using these roads: https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2516987122

Sometimes the shoulders appear dark (top image) and sometimes light (bottom). It only changes at game load and seems to be random. Once loaded, the shoulders are fixed. That is, they won't change during game play.

It only happens with these particular roads. I'm 95% sure there's some sort of mod conflict. Just hoping someone else may have seen this and knows.