How to change the nav bar background color on Android?

Hello I'm having issues with setting the navbar color with this:

var mainNav = new NavigationPage (new TodoListPage ());
mainNav.BarBackgroundColor = Color.FromHex ("a97946");
MainPage = mainNav;

This works great... until I push a Modal page then when I go back the navbar is the default color.
(this is happening on a Nexus 5 with Android 5.0, on my simulator running Android 4.4 does not happen)
is there a way to change the navbar color after setting it the first time?

Thanks!