//第一种 富文本
UIColor *color = [UIColor whiteColor];
_userName.attributedPlaceholder = [[NSAttributedString alloc] initWithString:@"用户名" attributes:@{NSForegroundColorAttributeName: color}];
//第二种 KVC 赋值
[_userName setValue:[UIColor whiteColor] forKeyPath:@"_placeholderLabel.textColor"];
