I have looked all through the How To Use the Discussion Board topics and FAQs and do find why some topic titles have and orange page icon with an arrow pointing to it. And I don't know why some topic titles are light green font, and others dark green font. Can someone help me understand these differences?

The orange page icon with an arrow next to a topic means that there is a new post in that topic since the last time you logged on (you'll only see these when you are logged in). The dark green/light green title font color changes happens after you've clicked on that topic - it jt just means that you've visited that topic during that logged in session.