How to change the TreeViewItem background Color on SelectedItemChanged event of TreeView?

You need to change the ItemContainerStyle of the TreeView in xaml. Bind the Background property to a string property in the item view model and use a converter to convert that string property into a Brush. You can then set the background from the view model when the IsSelected property is set. Here are some code snippets that may be useful.

<treeview.itemcontainerstyle>
      <style targettype="{x:Type TreeViewItem}">
       <setter property="Background" value="{Binding BackgroundColour, Converter={StaticResource BackgroundToColourConverter},Mode=OneWay}" />
       <setter property="IsExpanded" value="{Binding IsExpanded, Mode=TwoWay}" />
        <setter property="IsSelected" value="{Binding IsSelected, Mode=TwoWay}" />
       </style>
    </treeview.itemcontainerstyle>

 public class BackgroundToColourConverter : IValueConverter
   {
       public object Convert(object value, Type targetType,
    object parameter, CultureInfo culture)
       {
           string s = value as string;
         Color color;
           switch (s)
           {
               case "Green":
                   color = Colors.Green;
                   break;
               case "Red":
                   color = Colors.Red;
                   break;
               default:
                   color = Colors.White;
                   break;

           }
           return new SolidColorBrush(color);
       }

       public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
       {
          return null;
       }
   }