A low growl was what snapped Twilight back to reality but the sight of her marefriend crouching in front of her. The colour of her fur and mane beginning to fade into a darker hue and her mane began to straighten and falter. Realization crept into the alicorn’s mind as she whipped her head to the nearest window. The sun was no longer visible.

Basically, Pinkie senses danger and, since it’s after dark, she begins to allow Pinkamena to take over. Maybe it’s a quesadilla or Flash Sentry, all Pinkie knows is that she has to protect her marefriend. Twilight is just surprised by actually seeing this transformation.

I like drawing the Mane Six’s dark sides. I dunno, maybe listening to CreepyPastas and dark music helps with that. I might do something similar with Flutterbat and Lil Miss Rarity though. After testing of course.