+/- colors

What's the significance between the red +signs andthe grey +signs in the forum index?

Red + ... unread posts
Gray - ... you're up-to-date

I think Grey + (the original question!) is for (listen carefully I will say this only once)

forum that has sub forums where you have visited the forum, but some of the forums still have unread posts and you haven't visited them

(lies down in dark room)

mmcp42 is right, the "-" stands for categories that might have unread messages but that you have visited since the last new message arrive.

Looking at the forum index closer, it appears the grey + signs are all associated with the old forum only.

Red? It’s orange. Can’t people tell the difference between red and orange? Red is more like #FF0000 whereas this colour is quite clearly #FC6400.