Fair enough, you see, it starts with what the creators want to give to the players.
You start as a child, the colour white symbolises purity and innocence, but it's also matching with the brand that the company wants to sell to us.
For example, you're an artist, if you draw yourself as a "sky kid" the first thing that you'll do is to give to your character white hair, because it's recognisable, along with the combination of cosmetics that come from the game. If you see a sky kid with brown hair or red hair for example, it will be cool but can you really call it a "sky persona" ? A persona yes, but not a sky persona. The white hair sells better and it's more recognisable than anything else, therefore I doubt that they will want to change that, despite how cool and diverse this idea sounds like.