As a sighted person that has experience using Discourse I know what the background color signifies.
The question does bring up an important accessibility consideration. Using color alone to convey meaning is not best practice. eg. red for error messages.
For visitors that can’t see the color, maybe something non-visual could be added somewhere for a screen reader to pick up on?
For those that can see, they also have the “tone” of the post and its context within the topic to help them distinguish it from a peer post.
An analogy would be if a child picked up a toy and a book of matches. If the parent said “Put that back down” I think the child would know both that it was they that was being addressed and which item was being referred to.