修改 UITextfield 的 Placeholder字体的颜色方法如下:
[_textField setValue:[UIColor colorWithRed:0.2 green:0.2 blue:0.2 alpha:1] forKeyPath:@"_placeholderLabel.textColor"];
[_textField setValue:[UIColor colorWithRed:0.2 green:0.2 blue:0.2 alpha:1] forKeyPath:@"_placeholderLabel.textColor"];