//第一種 ?富文本

UIColor *color = [UIColor whiteColor];

_userName.attributedPlaceholder = [[NSAttributedString alloc] initWithString:@"用戶名" attributes:@{NSForegroundColorAttributeName: color}];

//第二種 ?KVC 賦值

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