Purple folders will appear for threads that you have never previously read but only in your first session(visit) to the site. If you spend 20 minutes, maybe 30 I forget, on one thread or typing a post and then go back to a different page in the forum all of the purple folders for threads that you have never visited will turn to plain.

The threads that you have visited/read in the past will properly reflect the purple/plain status. The other threads go to plain because the assumption is that you weren't interested in reading them the first time around, i.e. prior to your session timing out.

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