In Fallout Equestria: Project Horizons fanfic there's background character named Pastels. She's very little in the fanfic, just a few mentions, but I like how Somber describes her and her interactions with Blackjack.

...the dappled mare, Pastels,

...but Pastels was an artist,

And the most hilarious thing about Pastels for me was when Blackjack said that Pastels stole her crayon. She's still ticked off about it!

I mean, I was still ticked off at Pastels for stealing my crayon.

So, here's my idea for why Pastels stole Blackjack's crayon in the first place. And why it might be purple.