My strategy is to avoid conversations of this form entirely by default. Most Greens do not need to be shown that the belief system they claim to have is flawed, and neither do most Blues. Pay attention to what people do, not what they say. Are they good people? Are they important enough that bad epistemology on their part directly has large negative effects on the world? If the answers to these questions are "yes" and "no" respectively, then who cares what belief system they claim to have?