修改UITextfield的Placeholder字体的颜色

 //第一种

  UIColor *color = [UIColor grayColor];
    UITexiField *_userName.attributedPlaceholder = [[NSAttributedString alloc] initWithString:@"用户名" attributes:@{NSForegroundColorAttributeName: color}];


 //第二种
 [_userName setValue:[UIColor purpleColor] forKeyPath:@"_placeholderLabel.textColor"];