As in topic, really. Is it possible to search for posts based on whether they are marked as staff posts?
No that is not currently possible.
I would say this is more generally covered by the group page which lists all posts by that group, ala “blizzard developers” on the blizzard forums.
The reason I was after a “search for staff colour posts” instead was so that forum staff can freely be themselves (to an extent) but then signify that they are posting as staff by using the staff colour (which I assume is why staff colour is always an opt-in rather than defaulting to on for staff members). It would be useful for users to be able to see all “official” posts rather than getting bogged down in a mess of irrelevant posts sometimes.