Background mask does not change color

Adding color means adding an overlay. If you overlay color on top of black, then you will still have black. That means it is not possible to change a black background to color by using color overlay. What you can try is this. Use curves, and move up the lower left point of one (or two) of the three color curves. If you want a bright color, then also raise the exposure a bit. Your screenshot suggests you want to change the background to green, so move lower left point of the green curve upwards. I'm typing this on my iPad, so I can't show a screenshot from Lightroom Classic, but I mimicked it in Lightroom on my iPad.