有两个方法可以设置placeholder的颜色，代码如下：

//方法一
self.textField.attributedPlaceholder = [[NSAttributedString alloc]
            initWithString:@"Placeholder text"
                attributes:@{ NSForegroundColorAttributeName: [UIColor whiteColor] }];
//方法二
self.textField.placeholder = @"Placeholder text";
[self.textField setValue:[UIColor whiteColor] forKeyPath:@"_placeholderLabel.textColor"];