These days, I give green to people who do their best to facilitate a reasoned discussion. I also reward clever, funny, and of course, really great mainstream science explanations.
Too many people are so interested in "winning" an argument that they'll use really poor and obviously fallacious reasoning, so they get red from me. I give red to those who seem willful in their ignorance, asking for but then rejecting good science in favor of some pet idea. I give red to folks who don't understand the difference between critical analysis and personal attacks, and get rude about it. I give red to those who start with a decent question about science and then spend the rest of the thread trolling about how others treated their question.
I don't give red for wrong answers. I don't give red because someone disagrees with me. I don't give red usually unless the behavior was pretty blatant.
I give green to counteract someone else's red if I think it was done in retaliation for not agreeing with them. Those are always easy to spot,.