User blog comment:DiegoAnime3000/Black is for Polarity/@comment-3146930-20140110104132/@comment-11188061-20140110165836

It's not as simple as that. Weiss powered her weapon with greenish-yellow Dust during her fight in White Trailer, and it seemed to give her magnetism power. This is practically the same as polarity, so you'd wonder why it isn't black in color instead.