This is shown whenever you join the forum
The folder icon to the left of the forum is pink if there are new topics or replies that you have not seen
Once inside that forum you will see NEW next to the topic name if you have not looked at that page.
The only time you will see the folder icon in its beige colour if you haven't seen all the posts is if you have gone into that forum then left it without reading all the posts.
As soon as someone posts in that forum again the folder icon will change to pink