For every pony, color is a prominent aspect of their character, both in their mane and their fur. It colonizes their cutie mark, regardless of when, where, and how the symbol is granted. They're divided into Pegasi, Earth Ponies, and Unicorns, all of these qualities defining exactly who they are, and what they will be.

Except for one pony.

Lumina, born without color or a cutie mark, doesn't even have as much as an association, being neither a pegasus, unicorn, or earth pony. She literally appears like a customizable avatar, without any defining feature -- not a single gene from a predecessor or a single spark of magic flooding her bloodstream. In simplicity, she's tangible, but nothing at the same time. However, when a traumatic event unfolds in Canterlot, her destiny will slowly unfold, showing that she is indeed something in this massive, unpredictable universe.