There is a "legend" at the bottom of the page (list of threads ) that tells you what the colors mean. Something like this.
Contains unread posts
Contains no unread posts
Contains no unread posts
More than 250 replies or 4000 views
Hot thread with unread posts
More than 250 replies or 4000 views
Hot thread with no unread posts
Closed Thread
Thread is closed
Thread Contains a Message Written By You
You have posted in this thread