react-native-navigation/docs/styling-the-tab-bar.md

Styling the Tab Bar

You can style the tab bar appearance by passing a tabsStyle object when the app is originally created (on startTabBasedApp).

Navigation.startTabBasedApp({
  tabs: [ ... ],
  tabsStyle: { // optional, **iOS Only** add this if you want to style the tab bar beyond the defaults
    tabBarButtonColor: '#ff0000'
  }
});

Style object format

{
  tabBarButtonColor: '#ffff00', // change the color of the tab icons and text (also unselected)
  tabBarSelectedButtonColor: '#ff9900', // change the color of the selected tab icon and text (only selected)
  tabBarBackgroundColor: '#551A8B' // change the background color of the tab bar
  tabBarTranslucent: false // change the translucent of the tab bar to false
  tabBarTextFontFamily: 'Avenir-Medium' //change the tab font family
}

Note: On Android, add BottomTabs styles to AppStyle:

Navigation.startTabBasedApp({
  tabs: [...],
  appStyle: {
    tabBarBackgroundColor: '#0f2362',
    tabBarButtonColor: '#ffffff',
    tabBarSelectedButtonColor: '#63d7cc',
    tabBarTranslucent: false,
    tabFontFamily: 'Avenir-Medium.ttf'  // for asset file or use existing font family name
  },
...
}

All supported styles are defined here. There’s also an example project there showcasing all the different styles.