Can anyone advise me on how the border below the navigation bar can be changed?
I do not think there is a method to change the border color of the navigation color, other than the tintColor property of the UINavigationBar.
I would suggest that you create a UIView of that border size and place it below the navigation bar / add it as a subView.
UIView *navBorder = [[UIView alloc] initWithFrame:CGRectMake(0,navigationBar.frame.size.height-1,navigationBar.frame.size.width, 1)];
// Change the frame size to suit yours //
[navBorder setBackgroundColor:[UIColor colorWithWhite:200.0f/255.f alpha:0.8f]];
[navBorder setOpaque:YES];
[navigationBar addSubview:navBorder];
[navBorder release];