I think I started it. I've seen it used on other boards, and it's a nice way to identify Moderator posts, and gives us, as mods, the ability to post as "us" and as a Mod. To a slight degree, anyways.
I originally posted Moderator posts in
Red, as this is my color of choice. However, it contrasts poorly with Black, and was hard to read on the old boards. So I switched to a more Eye-strain friendly
Orange color. Most of the other mods have followed suit, and most chose to use orange, though as Redjack points out, each mod is free to use whatever color he wants, so long as it's consistent.
Mainly it's a tool to make moderator posts stand out a bit more, so that if someone's just skimming a long thread, they're more likely to notice it. And hopefully they won't skip over it without reading, and then do something we just told everyone not to do