BackGroundColor is a variable, not a property. So it only serves to read the work, but when you change it it does not generate a change in the view.
To change the background color you have to do it by design.
Or like this:
B4X:
MenuTree1.GetBase.Background=Colors.Red
Keep in mind that it is the second library that I created when I met B4A. So it has some flaws