How to change the appBar back button color

You have to use the iconTheme property from the AppBar , like this:

appBar: AppBar(
  iconTheme: IconThemeData(
    color: Colors.black, //change your color here
  ),
  title: Text("Sample"),
  centerTitle: true,
),

Or if you want to handle the back button by yourself.

appBar: AppBar(
  leading: IconButton(
    icon: Icon(Icons.arrow_back, color: Colors.black),
    onPressed: () => Navigator.of(context).pop(),
  ), 
  title: Text("Sample"),
  centerTitle: true,
),

Even better, only if you want to change the color of the back button.

appBar: AppBar(
  leading: BackButton(
     color: Colors.black
   ), 
  title: Text("Sample"),
  centerTitle: true,
),