Simple way to make default navigation bar transparent.
Just add these line to your AppDelegate class's didFinishLaunchingWithOptions function.
var imageForDefault = self.imageWithColor(UIColor(red: 0.0, green: 0.0, blue: 0.0, alpha: 0.0))
var imageForCompact = self.imageWithColor(UIColor(red: 0.0, green: 0.0, blue: 0.0, alpha: 0.0))
UINavigationBar.appearance().setBackgroundImage(imageForDefault, forBarMetrics: UIBarMetrics.Default)
UINavigationBar.appearance().setBackgroundImage(imageForCompact, forBarMetrics: UIBarMetrics.Compact)
Function I used to make an image is
func imageWithColor(color: UIColor) -> UIImage{
let rect:CGRect = CGRectMake(0, 0, 1, 1);
UIGraphicsBeginImageContextWithOptions(rect.size, false, 0);
color.setFill()
UIRectFill(rect)
let image: UIImage = UIGraphicsGetImageFromCurrentImageContext()
return image;
}