What you basically need to do is color the background of the selected node when the control loses focus.
You could try the following:
Private Sub TreeView1_LostFocus(sender As Object, e As System.EventArgs) Handles TreeView1.LostFocus
TreeView1.SelectedNode.BackColor = SystemColors.InactiveCaption
End Sub
Private Sub TreeView1_GotFocus(sender As Object, e As System.EventArgs) Handles TreeView1.GotFocus
TreeView1.SelectedNode.BackColor = Nothing
End Sub
Hope it helps :)