mark, the post id info turns red and stays red until the 48 hour mark, at which time, it turns black, but not a bold black like the posts less than 24 hours old.  So:

Post Age         Color of ID tag
0-24 hrs.         Bold Black
24-48 hrs.       Red
48  and after   Black

There were a few reasons to code the site like this, but the main was as a Mod tool.  A mod could quickly look at the thread and see if there were any new posts mixed in with the ones from the previous day as they would be bold black in a sea of red.  This was kind of outdated by the changes to the mod menu in 2009.

The 48 hr mark was picked because the threads were considered stale and were usually on the 2nd page and beyond after two days.

The founder actually explained this to me when I was training to be a mod, so it is not a theory.