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This is how I see it. The two dragons are just as easily influenced by these concepts as any human. Reshiram doesn't have to necessarily be on the side of pure truth, nor does Zekrom have to be on the side of pure ideals. It's how they are swayed by how strongly the hero feels about these concepts. Reshiram can side with N in one version because N attracts Reshiram with how true he believes his goal is, and vice versa with the other dragon and version. What it really comes down to, however, is that neither side is the right side because its a thin line between the concepts being the exact same thing. They feed off one another, hence the ying yang. Choosing one side is based on the person himself, because you wouldn't follow the dragon, they would follow you. If something is true in your heart, reshiram would follow, and if you place a lot of yourself into your ideals, your dreams your future endeavors Zekrom would follow. Unfortunately as proven in game, the dragons can be involved with the wrong side and it goes to show how manipulating the concepts can be when people feel strongly enough about them. The message of the game, however, is to balance both concepts. Just look at the hero twin brothers, they destroyed everything when they chose sides. That being said, the reason we don't know what the original dragon looks like is because it doesn't matter. Human nature isn't a perfect ying yang, we fall into categories. I myself am more on the Zekrom side based on my personality, but I wouldn't say reshiram is wrong either because like I said it's all up to interpretation.
