1) Can we disable that? I have no earthly idea where that is.
2) If not, can someone explain to be the rhyme or reason for why the colors are what they are? Is there some meaning to them?
The left border color on Post Boxes are unique per author. It's a way to visually associate each post with a specific person. This was explained in this blog post:
You can definitely remove the left border on those boxes using custom CSS.