Not sure if you’re looking for something different but you can change the background color via the scopeView property…
w = Window.new("my own scope", Rect(20, 20, 400, 500));
w.view.decorator = FlowLayout(w.view.bounds);
c = Stethoscope.new(s, view:w.view);
c.scopeView.background_(Color.blue);
c.scopeView.waveColors_( [Color.yellow, Color.red ] )
w.onClose = { c.free }; // don't forget this
w.front;