In flutter, the debug banner appears only on development mode and it is automatically removed when the app is in release build. Basically, a flutter debug banner is a small debug tag that appears on the top right. We can easily remove debug banner or debug tag by using the debugShowCheckedModeBanner property.
To remove debug banner in flutter add debugShowCheckedModeBanner: true property in MaterialApp.
MaterialApp( debugShowCheckedModeBanner: false, )
Also check, How to use hexadecimal color in a flutter.