This is an answer, rather than a question. Searching online, I only found a really hacked, contorted answer to this question (http://www.cocoabuilder.com/archive/cocoa/58379-cha
I just got a same problem.
To preserve original text attributes, my solution is here:
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];
}