Re: why xion's hair black
Let's not go down that rabbit hole.
I guess I should have said if Sora and Ventus were, for a better example, wearing identical clothes, then, without looking at their feet, could ou tell them apart.
I mean, sure you can tell Sora and Ventus apart. Sora has a white (or black) shirt and red (or black) pantaloons with big yellow (or black) shoes, and holds his keyblade right side up. Ventus has bland pants, regular sized shoes, and two fairly colorless shirts, with a shoulder pauldron, and holds his keyblade upside down.
More to the point, if Kairi, Namine and Xion were wearing ideintcal clothing and all had the same color hair, could you tell them apart?
One of the reasons Anime lends itself so heavily to cosplay is their use of iconic and brightly colored clothing which make the characters identifiable even though the artwork is very similar. It also makes it very easy to represent yourself as a character by dressing in that iconic clothing, with possibly a wig or trademark prop.
On the other hand, if you dress like Batou, people will just think you are some nutjob in a sheepskin jacket that spirit-glued a pair of silver goggle lenses to your eyes, especially when the supposedly non-toxic spirit glue makes your eyes start to burn and water and you are screaming and thrashing about because you can't see anything.
Or so I've been told...