Flutter Basics: My app bar color is not changing

running on my emulator using your code it works, try restarting your app completely.

import 'package:flutter/material.dart';

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

class RecipeApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final ThemeData theme = ThemeData();

    return MaterialApp(
      title: 'Recipe Calculator',
      theme: theme.copyWith(
        colorScheme: theme.colorScheme.copyWith(
          primary: Colors.grey,
          secondary: Colors.black,
        ),
      ),
      home: const MyHomePage(title: 'Recipe Calculator'),
    );
  }
}

class MyHomePage extends StatelessWidget {
  final title;
  const MyHomePage({this.title});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(title),
      ),
    );
  }
}

Try below code hope its helpful to you ,I think you can used Scaffold Widget refer AppBar class here and refer Scaffold class here

return Scaffold(
      appBar: AppBar(
        backgroundColor: Colors.grey,//change color on your need
        title: Text(
          'BottomNavigationBar Sample',
        ),
      ),
      body:Container(),//or your widget
);

Your result screen->