When Lily first appears to Rel Rel, she has purple hair, and a blue dress. However, this isn’t her true form. She appears to Rel Rel in a form that she is familiar with. Also, Rel sees things in her own way, and this is how Lily looks in her head.
This is an image of Lily’s true form… if it wasn’t made clear, based on the title. She is a Creator, one of the five races from the Muse Realm (where all imagination comes from).