Objection!

In the illustration from the book Twilight reads in the first episode, we can clearly see that, around 1,000 years ago, Celestia was an alicorn with a solid pink mane (and wings, oddly enough). Assuming that the illustration is accurate, at the time NMM was banished, Celestia was already an alicorn, but still had her pink mane!

…Wow, I got really hung up on that minor detail, didn't I?