I don't know if this is canon, or just false, but I believe that Luna looked like she does now before she was Nightmare Moon. She just might have been blasted to her original self after she came back. But if the picture does show one with pink hair, who is it meant to be other than Celestia? I've seen pictures of her fan-fiction parents (Some that you've drawn as well) and I didn't see any green in her genetics. Therefore, it would make more sence for her to have pink hair.