i'd say the skin color. But it's just the eyes and hair and figure. I guess I would always see her as what Juvia looks like. Just with bangs and other stuff. But Juvia just looks really similar in what I picture in my head as well.
About the last one... My problem is that I always start thinking which activity would be more time efficient so I can cram as many of them in my day as I can. BUT in the process I end up losing half of the time by wondering what to do.