Dayle,
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.  ;-)
Larry