Sorry if it's been mentioned before, but can someone explain to me the meaning of the colour coded envelopes [[red, blue & grey) on the threads?

Also, on the old forum, I could tell at a glance from the threads titles which ones I'd read [[they became shaded dark) from the ones I didn't read [[they stayed blue) - If a title became blue again I knew someone had posted to it - is there anything similar in place on the new forum, which i've missed?