说实话Google了一大圈，最后还是在Stack Overflow上找到了自己想要的答案，但是在swift里面还像没有imageWithColor方法，只能自定义了

下面是代码

func shadowImageLine() -> Void {
        self.navigationController?.navigationBar .setBackgroundImage(imageWithColor(UIColor.redColor()),forBarMetrics: UIBarMetrics.Default)
        self.navigationController?.navigationBar.shadowImage = UIImage()
    }
    func imageWithColor(color: UIColor) -> UIImage{
        let rect = CGRectMake(0.0,0.0,1.0,1.0)
        UIGraphicsBeginImageContext(rect.size)
        let context = UIGraphicsGetCurrentContext()
        
        CGContextSetFillColorWithColor(context,color.CGColor)
        CGContextFillRect(context,rect)
        
        let image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return image
    }