It is often, that someone adds many papers in JabRef. However, it''s difficult to keep track what you have read or not already. So, it could be that new entries (where new can be configurable) that have not been opened for more that 1 time (just when you add you may check that it works) can be marked with a star. Also, the rest of the entries can be colored according to their popularity (how many times they were opened.
Fixed and merged. Pull request #28
https://github.com/JabRef/jabref/pull/28