[onetwo_threefour]

I wouldn't use a mark, but I will disagree that sarcasm is always detectable. In many cases, people don't know you at all, and sometimes sarcasm or irony can be subtle enough that even people who do know you won't detect it. Having said that, the.only times I've used green text are when I think that the irony or sarcasm is subtle enough to be easily missed AND it's a situation where I don't want to be misinterpreted. In most cases, if you miss it, too bad for you.

BTW, I wouldn't use 'the Mark' because it really would ruin most instances and people would come to expect it every time, which eliminates some of the fun of using irony or sarcasm.