CustomWidget.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Column(
          children: const [
            MyWidget.red(),
            MyWidget.blue(),
            MyWidget.green(),
            MyWidget(
              color: Colors.pink,
            ),
          ]
        ),
      ),
    );
  }
}

class MyWidget extends StatelessWidget {
  // main constructor
  const MyWidget({required this.color});
  
  final Color color;
  
  // Contructores nombrados que asignan
  // la propiedad con un valor por defecto
  const MyWidget.red(): color = Colors.red;
  const MyWidget.green(): color = Colors.green;
  const MyWidget.blue(): color = Colors.blue;
  
  @override
  Widget build(BuildContext context) {
    return Container(
      width: 100.0,
      height: 100.0,
      color: color,
    );
  }
}