empty, no number, only a dot - occurs even for posts that have been voted
Posted recently.
a number in white
Not posted recently, and in Discussion.
a number in green
Not posted recently, and in Main.
I've occasionally seen green numbers in Discussion, but I think it happens when a thread has recently been moved to Main and parts of the database are still out of sync with each other.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.