导航栏设置的颜色由于系统在导航栏上面添加了蒙版的效果，所以设置上颜色值和实际上有一定的偏差

为了解决颜色上的偏差 创建一个继承自UINavigationController的类

- (void)viewDidLoad

 {

[super viewDidLoad];

[self.navigationBar setBackgroundImage:[[UIImage alloc]init] forBarMetrics:UIBarMetricsDefault];

[self.navigationBar setShadowImage:[[UIImage alloc]init]];

[self.navigationBar.layer.sublayers.firstObject addSublayer:[self gradientLayer]];

}

- (CAGradientLayer *)gradientLayer 

{

CAGradientLayer *gradientLayer = [[CAGradientLayer alloc]init];

gradientLayer.backgroundColor = [UIColor colorWithRed:28/255.0 green:166/255.0 blue:248/255.0 alpha:1].CGColor;

gradientLayer.frame = CGRectMake(0, 0, self.navigationBar.bounds.size.width, 20 + self.navigationBar.bounds.size.height);

return gradientLayer;

}