I was wondering about that too, honestly though it kind of felt like the mangaka ran out of time to color the hair maybe bc mana's hair was also uncolored 🤔I noticed that Allen's hair changed color on page 14. I guess the shock of Mana's condition made it turn to the white color we know.
Nea and Mana were originally one single being (The Millennium Earl) that opposed god, when that being died god split that soul in half and "cursed" them to continuously reincarnate as seperate beings without merging again.at some point I got lost in the whole Nea and Mana thing and now i think they're the same being and the poor guy is just bipolar. As for Allen i am confused, there was some ark with him when he was nursed by Cross after his mind was also broken, so this new ark with him and mana makes it even more confusing - supposedly Allen turned Mana into an Akuma, but Mana was never human instead was the millenary count, so isn't this some big mama plothole with the very beginning of the manga/ anime?