On the left panel, select Shapes, and pick the shape you want to use.
Resize it to fit the frame or if you're just using it as an effect, resize to your liking.
Under Settings, select the color you want to use.
You may also play with the Opacity, Animation and Blending settings.
Move the shape layer behind the main object by dragging it to the right.
Moving a layer to the left brings it "closer to the camera" and moving it to the right places it "farther from the camera".
By moving the shape to the right, you are placing it behind the main object which gives you a different background color.