Like Candi, I tend to use green font for most of my Out of Character (OoC) text, though I also try to be consistent in putting it in parentheses and prefaced with "OoC:"
I'm torn on this enhancement topic, as I both like and dislike the idea of a standard Out of Character (OoC) tag. I like the potential convenience, but my uncertainty stems from the fact that OoC comments can serve at least four different purposes:
* questions about a system ("What do I roll to do this?")
* questions about what's happening in the game ("Can my character see this from here?")
* declarations of actions with system-specific information ("I cast Magic Missile with a level 3 spell slot.")
* general commentary between players ("Good one, Jenny!")
...and it seems like they would be weighted and/or displayed differently. That third one - system-specific actions - needs to be particularly prominent. GMs often overlook this, so I put that text in red in many games. So I don't know if there's a "one implementation fits all" here; even with an OoC tag, there will probably continue to be conventions for which types of OoC comment will use the tag, and which will be presented differently.
In any case, I'll be interested in seeing a mock-up. It's entirely possible that one solution will fit most, perhaps all. I'll need to see it to judge, and probably even then, I'd have to try it in-game to see if it works better at attracting GM attention (necessary for the first three). I suspect a standard approach would work better, given that people would use it.
I realize it's a different topic, but this sort of begs the question about other conventions, and whether they should have standard tags. In-character speech is an example of such a convention, for instance. Most people on GP that I've seen use boldface and quotation marks to indicate in-character dialog. But not everyone does this; some use blue text, some use italics, some use quotation marks, some do a combination of these. Be prepared that if we get OoC tags, someone will ask for other text-formatting tags like 'In Character Dialog,' too.