This idea seems to be reminiscent of what is called a “color card” in other software, which is basically a layer whose only purpose is to give a background fill with a solid color (sometimes it enables gradients too).
To be fair, this could be worth implementing if we had compositing features, but there are two easier workarounds that do the same thing and depending on the situation could be more flexible.
- Create a bitmap layer and then fill the solid color yourself using the bucket tool.
- (my preferred one) Create a vector layer, draw a square outside the camera frame, and fill it with the bucket tool. This way you can use the palette global colors system, and change the color to your hearts content.
You don’t need to keyframe the color if you don’t want it to change, as keyframes have automatic exposures and will run until the end of the movie unless you add a new keyframe.
Since both solutions are key-frameable, you can change the BG Color at any moment by duplicating the keyframe and changing the colors.