Previously (on MyBB) I set up some tags [mod][/mod] which we could put around text, for example warning users to get back on topic. This would then surround the text in a big red box, turning the text to white. It was then crystal clear to users when they were receiving official instruction. It wasn't the ideal way to do it, as any users could actually use it if they figured out the tags.
Ideally, the software would provide the functionality for an Administrator or Moderator to specify all or part of their message content as "official". When this is then posted, it would highlight the specified text in some way.