主要代码：

bottomNavigationBar: BottomNavigationBar(
  //选中菜单颜色
  fixedColor: Colors.red,
  //图标大小，默认24.0
  iconSize: 30,
  //第几个菜单选中
  currentIndex: currentIndex,
  //当item数量大于3个时需要设置type属性
  type: BottomNavigationBarType.fixed,
  onTap: (index) {
    setState(() {
      currentIndex = index;
    });
  },
  items: const [
    BottomNavigationBarItem(
      icon: Icon(Icons.home),
      label: "首页",
    ),
    BottomNavigationBarItem(
      icon: Icon(Icons.category),
      label: "分类",
    ),
    BottomNavigationBarItem(
      icon: Icon(Icons.settings),
      label: "设置",
    ),
    BottomNavigationBarItem(
      icon: Icon(Icons.people),
      label: "用户",
    ),
  ],
),

注意： 当item数量大于3个时一定要加type: BottomNavigationBarType.fixed,属性，源码应该默认为了shifting属性，当大于3个时菜单显示不正常，但是可以正常点击。

不设置type属性或者设置了type: BottomNavigationBarType.shifting,未选中的显示为和背景颜色差不多相同的颜色了，仔细看还是能看到的。type属性设置为type: BottomNavigationBarType.fixed,时，显示正常。

其实不设置type属性也可以，只要给unselectedItemColor属性设置一下颜色就可以了。shifting只是一个移位动画，当大于3个item需要给未点击的item设置一下颜色值，要不然默认颜色会造成其他item看不清。如果设置fixed固定宽度或者item数量小于等于3时，不用给未点击的item设置颜色值用默认的就可以。

完整代码：

import 'package:flutter/material.dart';
import 'package:flutter_demo/tabs/category.dart';
import 'package:flutter_demo/tabs/home.dart';
import 'package:flutter_demo/tabs/people.dart';
import 'package:flutter_demo/tabs/setting.dart';

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

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(primarySwatch: Colors.blue),
      home: Scaffold(
        appBar: AppBar(
          title: const Text("Flutter"),
        ),
        body: const MyHomePage(),
      ),
    );
  }
}

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

  @override
  Widget build(BuildContext context) {
    return const Tabs();
  }
}

class Tabs extends StatefulWidget {
  const Tabs({super.key});

  @override
  State<Tabs> createState() => _TabsState();
}

class _TabsState extends State<Tabs> {
  int currentIndex = 0;

  List<Widget> pages = const [
    HomePage(),
    CategoryPage(),
    SettingPage(),
    PeoplePage()
  ];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: pages[currentIndex],
      bottomNavigationBar: BottomNavigationBar(
        //选中菜单颜色
        fixedColor: Colors.red,
        //图标大小，默认24.0
        iconSize: 30,
        //第几个菜单选中
        currentIndex: currentIndex,
        //当item数量大于3个时需要设置type属性
        type: BottomNavigationBarType.fixed,
        onTap: (index) {
          setState(() {
            currentIndex = index;
          });
        },
        items: const [
          BottomNavigationBarItem(
            icon: Icon(Icons.home),
            label: "首页",
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.category),
            label: "分类",
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.settings),
            label: "设置",
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.people),
            label: "用户",
          ),
        ],
      ),
    );
  }
}

home.dart

import 'package:flutter/material.dart';

class HomePage extends StatefulWidget {
  const HomePage({super.key});

  @override
  State<HomePage> createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  @override
  Widget build(BuildContext context) {
    return const Center(
      child: Text("首页"),
    );
  }
}

其他子布局代码和home.dart代码类似。

BottomNavigationBar更多属性请参考源码：

BottomNavigationBar({
    super.key,
    required this.items,
    this.onTap,
    this.currentIndex = 0,
    this.elevation,
    this.type,
    Color? fixedColor,
    this.backgroundColor,
    this.iconSize = 24.0,
    Color? selectedItemColor,
    this.unselectedItemColor,
    this.selectedIconTheme,
    this.unselectedIconTheme,
    this.selectedFontSize = 14.0,
    this.unselectedFontSize = 12.0,
    this.selectedLabelStyle,
    this.unselectedLabelStyle,
    this.showSelectedLabels,
    this.showUnselectedLabels,
    this.mouseCursor,
    this.enableFeedback,
    this.landscapeLayout,
    this.useLegacyColorScheme = true,
  })...