How to change NavBar colour of MasterMainPage in Xamarin

Here, when you assign App.xaml's MainPage, a NavigationPage, it shows it's own NavigationBar. Under the hood, your MasterDetailPage also shows the NavigationBar. Thus, you are viewing two NavigationBars.

Go to your MainPage.xaml.cs backend page and in the Constructor, write the line:

NavigationPage.SetHasNavigationBar(this, false);

Thus, your MainPage.xaml.cs should look like :

public MainPage()
{
    NavigationPage.SetHasNavigationBar(this, false);
    InitializeComponent();
    ......
}

This will hide the NavigationBar of MasterDetailPage.