BG-color is a background effect on Powder Game 2 implemented in ver7.6. The background color, usually black, can be changed by the player when this BG is selected. This allows for backgrounds, such as a sky, for their upload without the use of several joints and joining all visible elements. However, the background spans the entire screen, so joints will have to be used for any additional sub-backgrounds.
The color of the background is decided by which elements on the menu are left- and right-clicked. Their RGB components are averaged, and the resulting color is shown. Anything on the menu that is not an element is assumed to give the game's traditional black color.
If the left- and right-clicks are on the same element, and if that element is somewhere on the screen, its color changes to black to keep it from becoming completely invisible.
Interestingly, if two different elements are selected to make the equivalent of a third element (for example, left-clicking Stone and right-clicking Metal produces the same color for Block), that element blends in and does not change to black. The same happens with joints.
Sometimes in an upload, the element(s) the uploader selected are the element(s) selected when the viewer first sees the upload, no matter the BG. If the upload was set on BG-color, however, the two elements selected are guaranteed to be selected when the upload is first seen by the player, since the uploader probably wanted that color combination to appear in the upload as the background. If the uploader selected a tool instead of an element (to make the background color darker), the intended color is shown on the thumbnail in the Save Data menu, but the game resets the selection to either Powder or Wind (depending on whether it was left- or right-clicked) when it is viewed.