I'm trying to set a `StatefulBuilder` widget state outside of its widget. Most examples and the documentation available show the `setState` method being used inside the widget, however I'm wondering if such method can be called from outside of the `StatefulBuilder` widget. Here's an example:

```
import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'StackOverflow Example',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'StackOverflow Example'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return new GestureDetector(
      //Change one of the icon's color using the tap down function
      onTapDown: (TapDownDetails details) {
        return changeColor(details);
      },
      child: Scaffold(
        appBar: AppBar(
          title: Text('Example'),
        ),
        body: Center(
          child: Column(children: [
            //This widget should be rebuilt
            StatefulBuilder(
                builder: (BuildContext context, StateSetter setState)
                {
                  Color _iconColor = Colors.black;
                  return Icon(
                    Icons.money,
                    size: 50,
                    color: _iconColor,
                  );
                }
            ),
            //This icon should not be rebuilt
            Icon(
              Icons.euro,
              size: 50,
              color: Colors.black,
            ),
          ]),
        ),
      ),
    );
  }

  void changeColor(TapDownDetails details) {
    //Rebuilt StatefulBuilder widget here, but how?
    setState(() {
      _iconColor = Colors.green;
    });
  }

}

```

Currently I get an error because of the `_iconColor` variable being used in `setState`. I am also aware that it may be impossible to access it outside of the widget. If that's the case, what would be a better solution to change the icon's color without resorting to rebuilding the whole `StatefulWidget`?

Thanks for your time.