Flutter - How to change TextField hint color?

As an addition to the accepted answer, to update the focused hint decoration you have to update the app's Theme.

@override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
          primaryColor: Colors.white,
          inputDecorationTheme: const InputDecorationTheme(
            labelStyle: TextStyle(color: Colors.black),
            hintStyle: TextStyle(color: Colors.grey),
          )),
      home: MainScreen(),
    );
  }

You can do with hintStyle: in InputDecoration

TextField(
        style: TextStyle(fontSize: 20),
        decoration: InputDecoration(
          hintText: "Password",
          hintStyle: TextStyle(fontSize: 20.0, color: Colors.redAccent),
          border: OutlineInputBorder(
            borderSide: BorderSide(
              color: Colors.teal,
            ),
          ),
          prefixIcon: const Icon(
            Icons.security,
            color: Colors.white,
          ),
        ),
      ),