I got inspired by some other Bronies who've headcanoned that Celestia and Luna's mane colors are influenced by the celestial bodies they raise/lower so I illustrated my own take on that.

I personally go with the fanon idea that Celestia was born with an all-pink mane and tail, but she would have gained more warm sun-themed colors like oranges and yellows in her hair as she got older. But after banishing Nightmare Moon and having to raise BOTH the sun and moon on her own, that's how Celestia also gained cooler colors that one would associate with the moon

The same thing would have applied to Luna in my headcanon. Her coat colors + hair have always been consistently themed with the moon/nighttime colors since all her life she only ever had to be in charge of raising/lowering the moon. But IF in another universe Luna had to also raise the sun AND moon for any reason she would have gained more colorful, aurora/celestial-themed colors in her hair

Also I do not care that this doesn't potentially line up with what the show or comics or G5 may have implied, this is just how it works in my universe