设置textfield的placeholder的颜色

//第一种

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

//第二种(不建议使用,调用私有方法上线审核时容易被拒)

[_userName setValue:[UIColor whiteColor] forKeyPath:@"_placeholderLabel.textColor"];