How to access color from App.xaml in C# in Xamarin Forms?

In App.xaml I have this code:

#ffd966

and in C# I have this code:

public Color BackgroundColor
{
    get { return IsSelected ? Color.Yellow : Color.White; }
}

And I would like to change Color.Yellow with color from App.xaml. How can I reference color from App.xaml in C#?