[JJR512]

The gray Js (actually, I think of them as silver) indicate there are new posts. The dark blue Js indicate no new posts. I'm sure there's a legend there somewhere, I'll have to point it out to that member.

...Yeah, there actually is a legend on the main forums page, that says this:
New Posts
No New Posts

I think if you saw the colors of my forums (same colors as here, actually), you would agree the silver J icon stands out more, and thus is a better choice for ON than the blue icon, which blends in.

If you want to see those links, try looking at a category (like Computer Stuff of General), or at a forum with subforums (like Hardware or Mindless Blabbering).