envelope icon?

Can someone please explain the different envelope icons? Sometimes they are light blue, then dark blue, then light red, then dark red. Does this have to do with which ones you've read, or posted in, or something like that?

duh, nevermind I see it at the bottom now. I must remember not to drink while thinking about conspiracies.

[Edited on 11-18-2003 by JFetch]

It explains it at the bottom of the page.