main.dart

final repo= FetchColorsFromImage();

// i have used networkImage you can use any ImageProvider class for it

var image=NetworkImage(<imageurl>);
Color backgroundColor =Colors.black;

void initState(){
   getColors();
   super.initState();
}

getColors() async {
setState((){
  backgroundColor=await repo.getColor(image);
});

}

.....
 return Scaffold(
   backgroundColor:backgroundColor,
  ....
);