How to implement colours dropdown in flutter?

try this way

import 'package:flutter/material.dart';

const Color darkBlue = Color.fromARGB(255, 18, 32, 47);

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

class MainApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(
        scaffoldBackgroundColor: darkBlue,
      ),
      debugShowCheckedModeBanner: false,
      home: const Scaffold(
        body: Center(
          child: MyApp(),
        ),
      ),
    );
  }
}

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

  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  Color pickColor = Colors.red;
  List colorsList = [
    Colors.red,
    Colors.yellow,
    Colors.black,
    Colors.pink,
    Colors.blue
  ];

  @override
  Widget build(BuildContext context) {
    return Container(
      padding: const EdgeInsets.symmetric(horizontal: 10, vertical: 10),
      decoration: BoxDecoration(
          color: Colors.white, borderRadius: BorderRadius.circular(8)),
      child: DropdownButton(
        style: const TextStyle(color: Colors.black),
        //Dropdown font color
        dropdownColor: Colors.white,
        //dropdown menu background color

        //dropdown indicator icon
        underline: Container(),
        //make underline empty
        value: pickColor,
        onChanged: (value) {
          pickColor = value! as Color;
          setState(() {});
        },
        items: colorsList.map((itemone) {
          return DropdownMenuItem(
              value: itemone,
              child: Row(
                children: [
                  const Text(
                    'Color: ',
                    style: TextStyle(
                        color: Colors.black, fontWeight: FontWeight.w700),
                  ),
                  Container(
                      height: 20,
                      width: 20,
                      decoration: BoxDecoration(
                          borderRadius: BorderRadius.circular(20),
                          color: itemone))
                ],
              ));
        }).toList(),
      ),
    );
  }
}