Click "Details" in the download window when it is green and of the download window when it is yellow. Look at the "announce url." It will look sort of like:
http://tracker.somesite.com/announce
After the tracker url's address ("tracker.somesite.com") and before the path ("/announce"), there will be a colon (":") and a port number ( such as "6969"). A sample tracker site with a port number looks like:
http://tracker.somesite.com:6969/announce which is an example of a tracker on port "6969." If the tracker doesn't have a colon and a port number between the site name and the path (looks sort of like this:
http://tracker.somesite.com/announce), then it is on port 80.
Now, knowing how to identify the ports, do you see any pattern in the tracker's ports with respect to a certain download status? Are the ones you are yellow with on download on a specific port, like port "80" or "6969" ? Are the ones you are mostly green with on a specific port? Does it happen regardless of the port?