They are two printed card stock backdrops which were first laminated and then cut out to remove the sky on each which was a different shades of blue from the other. Total cost plus lamination about $26.00. I also did the same on the other side of my layout in a corner with a paper mill. They are stuck on the wall with poster putty.
I do have some corner stone building fronts, several OGR building fronts, and etc., but they are (obviously) not visible from this vantage point.