在网上搜索，我只找到了a really hacked, contorted answer to this question，可以更优雅地回答如下：

NSArray *itemArray = [scalePopup itemArray];
int i;
NSDictionary *attributes = [NSDictionary
                            dictionaryWithObjectsAndKeys:
                            [NSColor redColor], NSForegroundColorAttributeName,
                            [NSFont systemFontOfSize: [NSFont systemFontSize]],
                            NSFontAttributeName, nil];

for (i = 0; i < [itemArray count]; i++) {
    NSMenuItem *item = [itemArray objectAtIndex:i];
    
    NSAttributedString *as = [[NSAttributedString alloc] 
             initWithString:[item title]
             attributes:attributes];
    
    [item setAttributedTitle:as];
}

我也有同样的问题。
为了保留原始文本属性，我的解决方案如下：

NSRange range = NSMakeRange(0, 0);
NSAttributedString *cellStr = [[self.popup cell] attributedTitle];
NSMutableDictionary *cellAttr = [[cellStr attributesAtIndex:range.location effectiveRange:&range] mutableCopy];
[cellAttr setObject:[NSColor redColor] forKey:NSForegroundColorAttributeName];

NSArray *menuItems = [self.popup itemArray];    
for (NSMenuItem *menu in menuItems ) {
    NSString *orgTitle = [menu title];
    NSMutableAttributedString *title = [[NSMutableAttributedString alloc] initWithString:orgTitle attributes:cellAttr];

    [menuItem setAttributedTitle:title];
}

Swift 5.5

为菜单项分配属性字符串。例如，如果以前选择了该菜单项，则通过分配粗体绿色属性字符串来更改标题颜色。
我有一个属性，我为以前选择的标题，所以我可以很容易地删除属性字符串，如果另一个标题被选中。

var previousTitle: String = ""

然后在我的方法中，我做以下事情

if !previousTitle.isEmpty {
    // Remove the previously assigned attributed title
    myPopUpButton.selectItem(withTitle: previousTitle)
    myPopUpButton.selectedItem?.attributedTitle = nil
}
previousTitle = <some previously selected title>
myPopUpButton.selectItem(withTitle: previousTitle)
let font = NSFont.systemFont(ofSize: 13).bold
let foreground = NSColor.green
let attrString = NSAttributedString(string: previousTitle, attributes: [NSAttributedString.Key.font: font, NSAttributedString.Key.foregroundColor: foreground])
myPopUpButton.selectedItem?.attributedTitle = attrString