[Kittie Rose]

Please note that Japan IS a very conformist society, so conformist it actually freaks you out. So unless you don't want to work in those "normal" 9-5 jobs that ensures some sort of financial income in Japan, it's better off for you to keep your hair undyed.

I don't think Japan is any more comformist than here, and is probably less so even. I live in a very conservative country. Just about every set of "Look I've been to Japan!" captures any number of oddballs. You just wouldn't find any where I live, at all.

And you do realize the story is set in Japan, where people's NATURAL hair colour is black? (Or darkish brown at best. )

Doesn't matter, Japanese people dye their hair too. Misa is a model, a gothic lolita model. Many of them have bleached blonde hair, singers too. Like Kana who changes her hair every other week, but is generally blonde.

You've said it yourself, most mangaka and anime creators draw their characters with colourful hair probably because black hair is boring. But I must emphasize again that this is a LIVE ACTION adaptation.

So? It doesn't just look more interesting in cartoons, it looks more interesting IN GENERAL. If anything, notable hair colours are more interesting in real life because you expect bright colours in comics and cartoons.

the audience would prefer to see their characters as "realistic" as possible in order to better suspend their disbelief.

I don't know how much this is true. I think it's one of those things that people believe because it sounds like it must be true, but isn't. In reality there are things even worse than X-men that people accept. Lord of the Rings for instance, heh. And a lot of godawful decisions are made in the name of it. See: Transformers movie.