So, I've seen some fan made videos, the toys, plushies, comics, and the show, and I have to wonder about why Celestia, who used to apparently be white with pink-coloured mane/tail, would now be her sparkle-butt rainbow colours... my guesstimate:

1: Since using the Elements of Harmony to seal her sister in the moon, alone and using only her own willpower, it dyed her follicles in her mane and tail, immediately, permanently.
2: Since having sealed her sister away, over time, a good hundred years plus, using two kinds of magic, solar and lunar, she has mixed her magics, thus changing her solar magic, appearing more pink and yellow, to remixing in the darker shades of the spectrum of her sister's magics, of blue and black, creating the light blue and green in her colours.


This also allows for the weakening, furthermore, for Luna to kidnap/imprison her sister during the Summer Sun celebration, season 1 episode 1, with the mixing of magics, it makes Celestia much more susceptible towards Luna's tricky magics. Luna is, by far, the far stronger of the siblings, being able to fight her sister, without assistance, the very first time she was banished, until Celestia grasped upon the Elements of Harmony.