My guess is that it's just not concentrated enough to be obvious in solution, and has been slowly depositing on the tube walls.  In that second pic it's obvious there's a tint, but it wasn't actually visible to me at that point.  I pondered then why my pretty tubing was showing up so badly in the pics. Over the next few days, more material oxidized and/or built up on the tube walls, making it more apparent visually.

Right now, visually the appearance is close to the second pic in my post above, but to the camera it looks like someone shat in the tank: