Players are able to change the color of the brushes on any given map. As seen in the screenshot, the given minge/exploiter changed the grass to a more green color. However, I have seen transparency, blue, orange, all kinds of crazy colors. The color itself does not matter, apparently.
The exploit is able to change the grass/skybox color to any color, even a transparent one, causing the “hall of mirrors” effect.
It appears the exploit is, erm, exploited through expression 2 in some way. No other tools or methods of interaction have been able to cause this effect.
I’d be happy to provide extra info if necessary. I have no idea how to make an image a thumbnail, but here you go. http://img97.imageshack.us/img97/7554/gmflatgrassdaynight0077.jpg