I know that in episode S4 parts 1 and 2 we see how celestia banished nightmare moon. But what if they actually only looked like their present selves in twighlight's point of view? What if back when celestia actually banished luna, her hair was pink like in the book in the beginning? Could it be that when she used the elements of harmony her hair changed colors from the blast? It would make sense seeing as the colors of the elements of harmony look a lot like the colors in celestia's hair? I don't know.....