"Blues and Greens" is a term used to metaphorically refer to opposing political factions.
See also: Tribalism, Mind-killer, Arguments as soldiers, False dilemma, In-group bias
The term come from the names of chariot racing teams, that differed in nothing but the team colors, but the rivalry of whose fans sometimes reached the level of gang wars.1 By definition, politics also deals with matters that people physically fight over in the real world -- or at least, matters that are to be enforced by the government's monopoly on violence.
Politics commonly involves an adversarial process, where factions usually identify with political positions, and use arguments as soldiers to defend their side. When tempered by appropriate standards of evidence, rules of order and other safeguards, such a process may be the only way of introducing a modicum of deliberative truth-seeking and other virtues of rationality into an inherently violent domain. However, the dichotomies presented by the opposing sides are often false dilemmas, which can be shown by presenting third options.
For a variety of reasons, Less Wrong tries to avoid political disputes: see Mind-killer.