I agree that the green-->yellow change happens for new threads (or topics, as they call them). But within a topic, there doesn't seem to be a similar coding of individual replies.
In other words, even though the topic may be yellow (I've read the topic before), there may or not be new postings that I haven't read. The only way to find out is to go look at the topic and re-read the entire thread.
Oh well. Nothing is perfect -- except me, of course. ;-)