@wolfeng I think it's probably more along the lines of "Most people with high mana have black hair." rather than "If you have high mana you must have black hair." Just so happens that she has pink hair and also a lot of magical power.
@Svelandria Its actually that black hair is EXCLUSIVE to high magic. Which means to have black hair you need high magic. Its not that to have magic you need black hair but rather the other way around.
This means its not at all a 1:1 ratio since no one said people with high magic have black hair.
@Merilirem Perhaps I'm misunderstanding your post, but I believe that you and I are making similar arguments. "Most people with high mana have black hair" meant that there are people with a high amount of magic, and most of them have black hair(both Leonard and the girl clearly have a lot of magic, but only Leonard has black hair). With that said, I wasn't aware that black hair was exclusive to high magic, so I did originally assume that there might be people with black hair who don't have high magic. Also, I knew it wasn't 1:1, but to be clear I don't know the actual ratio, so in hindsight I shouldn't have used the phrase "most people" as it could very well be that few people with high magic actually have black hair. For all I know, it could be that 1 in 10 high magic users actually have black hair, which would invalidate some of my argument. Wouldn't be the first time I was wrong. ¯\_(ツ)_/¯