>>42310077
White light splits into all colors. They can also combine back into white light

I think Celestia is rainbow themed to imply she has ALL the best traits of the mane 6/EoH, combined. She's supposed to be the best of all of them, the ultimate mare of harmony. And the ONLY individual who has ever used the Elements all by herself, because she's the only one who strongly represents each Element simultaneously

>protective motherly instincts, honesty and humility like Applejack. also workaholic tendencies/wanting to handle all the burden so no one else has to
>Pinkie's love of festivity, sweets, making others smile, and having a bit of a playfully teasing side. she can indulge in spontaneous fun sometimes.
>rarity's fanciness and love of luxury, cleanliness, and beauty
>twilight's curiosity and perpensity for introverted bookish stuff
>fluttershy's compassion and soft (but strong) heart. her love of mother nature. her gentleness
>rainbow's instincts to protect others from grave danger, her don't-fuck-with-me attitude when the situation calls for it, a pinch of RD's fiery nature.....and of course her RAINBOW mane

Celestia has it all. She is the ultimate mare.