随机生成一个RBG颜色

-(UIColor *) radomColor{

    CGFloat red = arc4random_uniform(255) / 255.0;

    CGFloat green = arc4random_uniform(255) / 255.0;

    CGFloat blue = arc4random_uniform(255) / 255.0;

   return [UIColor colorWithRed:red green:green blue:blue alpha:1.0f];

}