It's certainly against the MUI styleguide to hard-code background colours because you don't know the MUI settings. The Area.Background tag should work, though. Thore?
I cannot tell why the custom background is applied to the wrong object, but all subclasses of List.mui set their background internally to either MUII_ListBack or MUII_ReadListBack. As Andreas already said this is something that should not be changed, as it contradicts the consistency of what the user has configured. However, if a custom background is really required then it is possible to set a different one after the object has been created. But you must make sure to set the background for the list object and not for a possibly existing surrounding listview object. With MUI4 Listview.mui is no longer required to display the embedded list and if it is used all attributes and methods of Listview.mui are directly passed to the embedded list object. But this does not apply to general attributes of Area.mui like MUIA_Background.
But you must make sure to set the background for the list object and not for a possibly existing surrounding listview object.
Ok, then it's not possible with MUI Royale because it always uses the listview object when calling any methods or using Set() or Get().
Edit: Since djrikki might wonder why we're talking about listview objects here at all: That's how Floattext is implemented in MUI. A floattext object is basically a read-only listview object.