First of all, my theory is based around the whole "xion is soras memory of kairi" theory. I belive this theory to be true because of her hair. Since 358/2 takes place before CoM, sora's memory of kairi, her hair is this way. I'll get to why I think its dark later.
Now, what happened to xion takes place in CoM. As sora forgot kairi, xion existence slowly faded away.
My next theory is about why Org.XII is still called this with XIV members. As the memories of kairi slowly faded away from sora, as I said before, xion slowly faded away as well. But, as stated by namine and Leon, as soras memory faded, all people connected to him had there memories fade away. So, what I think is that this also affected th Organizations memory to. So, I belive they were called Org.XIV, then, as sora memory faded, xions exicestence also faded. Did the organization know this was going to happen, yes. they did it because xion disobeyed them so badly that they wanted to erase her exictence completely.
So, heres why her hair is dark. It's because she is also the memory of aqua. Why, because when sora was little, he and aqua hearts connected, thus keeping her memory in sora. I also think xehanort and aqua met, but when he became a nobody, he forgot aqua. but, when sora forgot kairi, xemnas remembered aqua.
Now, what happened to xion takes place in CoM. As sora forgot kairi, xion existence slowly faded away.
My next theory is about why Org.XII is still called this with XIV members. As the memories of kairi slowly faded away from sora, as I said before, xion slowly faded away as well. But, as stated by namine and Leon, as soras memory faded, all people connected to him had there memories fade away. So, what I think is that this also affected th Organizations memory to. So, I belive they were called Org.XIV, then, as sora memory faded, xions exicestence also faded. Did the organization know this was going to happen, yes. they did it because xion disobeyed them so badly that they wanted to erase her exictence completely.
So, heres why her hair is dark. It's because she is also the memory of aqua. Why, because when sora was little, he and aqua hearts connected, thus keeping her memory in sora. I also think xehanort and aqua met, but when he became a nobody, he forgot aqua. but, when sora forgot kairi, xemnas remembered aqua.