It's a unicorn with the ability to grow back his/her horn that lives in a fantasy world. Such is the case, it CAN be purple with green polka dots. I've always thought he/she was metallic gold based on the coloring in the comics. I have a custom in the works and I have to say, the metallic gold looks very cool on it.
Being a fantasy doesn't mean anything at all goes. I gave a reason, it'll look too inorganic, and that's a good enough reason for me to not use that color on the body.