Cannot change Navigation page barbackgroundcolor programmatically

Hi
Wondering if is possible to change the navigation bar background programmatically
without using renderers.

I have an app that uses prism not that matters .
I have a masterpage and in my app.xaml I have the following

     <Style TargetType="NavigationPage">
        <Setter Property="BarBackgroundColor" Value="OrangeRed" />
        <Setter Property="BarTextColor" Value="Blue" />
      </Style>

so far so good!!!

Now when I go to a particular page I need to change the BarBackgroundColor.

Whatever I tried did not work(below code anywhere is always null!!)

        //does not work as navigationPage is null
        var navigationPage = App.Current.MainPage as NavigationPage;
        navigationPage.BarBackgroundColor = Color.Red;


I even created a class :MyNavigationPage and injected it and still does not work

public class MyNavigationPage : Xamarin.Forms.NavigationPage, INavigationPageOptions, IDestructible
{
    public MyNavigationPage()
    {

    }
}
     private readonly myNavigationPage navPage;

    public MenuViewModel(INavigationService navigationService,
    MyNavigationPage navigationPage) : base(navigationService)
    {
        this.navPage = navigationPage;
    }
    private void OnMenuItemTapped(MasterMenuItem menuItem)
    {
       navPage.BarBackgroundColor=Color.Red;
    }

    }

any samples out there where it works!!!
Is this a bug?

many thanks