I haven’t figured out what the point of ‘staff color’ is, since it can be applied to non-staff posts.
What this site needs is some kind of glossary and conventions page, to define terms that might not be industry standard ones and conventions that Discourse or its user base have found helpful over the years.
i rewrote the Discourse moderator guide for our site and it has a section on staff color. it is to indicate a formal staff post at any time in a topic. say a topic discussion is getting heated and a mod wants to step in with a post that is an official staff post to cool it off, they can use the staff color to indicate this. i use it more than my mods for when i want to highlight a tech or admin related post. today i used it to post an official happy birthday topic to a very longtime well-liked user who is very old. another similar tool is the staff notice, which ads an official staff notice statement header above any post.