如果你想要一个固定的图标颜色，你可以使用这个：

TextField(
  decoration: InputDecoration(
    prefixIcon: Icon(Icons.done, color: Colors.green),
  ),
),

Github上有关于此的问题

更新：

看到这段代码了吗

class MyHomePage extends StatefulWidget {
  MyHomePage({Key? key}) : super(key: key);

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

class _MyHomePageState extends State<MyHomePage> {
  FocusNode _focus = FocusNode();

  //Define your default color  
  Color fieldColor = Colors.green;

  //Using focus node everytime the focus change the listener will change the 
  //color and call setState
  @override
  void initState() {
    super.initState();
    _focus.addListener(() {
      setState(() {
        fieldColor = _focus.hasFocus ? Colors.red : Colors.black;
      });
    });
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: TextField(
          focusNode: _focus,
          decoration: InputDecoration(
            prefixIcon: Icon(Icons.done, color: fieldColor),
          ),
        ),
      ),
    );
  }
}

我可以更改TextField的 Package ：

Theme(
      data: Theme.of(context).copyWith(primaryColor: Colors.black),
      child: TextFormField(),
    );