If you scroll down to the bottom of the main page of each sub-forum, there's a list of all the icons and what they mean. The color shows that a thread has new posts since you last visited, and when they're flashing it shows the more popular threads with lots of posts.
The icon list is pretty easy to find once you know where to look, but it's not on every single screen.
Hope this helps!
Dx'd with MS & HNPP (hereditary peripheral neuropathy) 7/03 but must have had MS for 30 yrs before that. I've never taken meds for MS except 1 yr experiment on LDN. (I found diet, exercise, sleep, humor, music help me the most.)