What if someone insists on self-describing as a Blue, but most Blues disagree with him and say he is completely confused about what Blue-ness is?

Sometimes there are many tinges of Blues. And for almost every tinge you pick, most other Blues will claim people of that tinge are not really Blue. (Religious and ideological movements get like this a lot.) But Greens have no problem classifying people as Blue and non-Blue, so it's not a wholly useless concept.