Kairi's--and a lot of the cast, really--hair seems to change color often, tbh. Sometimes it looks red; sometimes it looks brown or black, but really I think it's a mix of all three of those colors. Lighting is also something to factor into the argument.
That, and the fact that her hair looks different in CG than it does otherwise. For instance, when Sora's reading the letter from Kairi at the end of II, her hair almost looks a bit orange (but I think that was because the sun was setting in the background). But right after, when Sora and Riku return home, her hair looks really red (and for that matter, her outfit looks more baby pink there than it does coral). It's never that red usually, so whether or not the scene is pre-rendered or in real-time also plays a part in her ever changing hair... I've video edited with the KH scenes a lot. Leave me alone. LOL.
In any case, her hair seems to be more red than anything else in the official art, so I guess that's really what it's supposed to be.
Also, I think the may or may not have made child!Kairi's hair a bit browner in the remastered BbS scenes with her.
And on a completely unrelated note, I think I read in this article (on another site) that they said there was a new reward for beating the game. Are they talking about the custom theme you get or something else that's new?
