Ha, so I finally reawakened my Lifestream account (always thought I would for LTD reasons, actually). Anyway~ I'm the Wiki user WhiteArcticWolf, and all I can really say is that the creators must have changed it, like they often do with designs/storyline elements. Tifa has brown hair in the original and BC, and those were the two early works. In her AC profile she has black hair and she appears with black hair in CC as well (I even just checked it to make sure). Black hair does sometimes have a brown-ish coloring in the light, which accounts for why it looks dark brown sometimes.
Still, I don't get why they wanted to change it. Did they think she looked prettier or wanted to differentiate her from Aerith? Or is it one of those unexplained "Let's make it this way" things? Eh, if there's ever a remake, though, Tifa's hair will probably changed to coincide with AC/CC. For some reason I'm reminded of Rufus' orange juice colored hair that he had for FFVII, except for, like, one scene...Even though his concept art had him as a blond.