Mira(ge) is a changeling. She can change her form at any time.

When her hair is brown, she's normal. But if it's pink and green, she's in her changeling form.

So this is what happened;

Ghost, susceptive of Mira being a ghost, dumps a bucket of ice cold water on her. Out of shock and hurt, she begins to revert back and forth between her two forms.

A glitch in her transformation.

"Wh-Why would you.. Do that..?"