the reason is that new maps are still getting unranked over this rule
I'm done here. Talking with staff of their mistakes gets you nowhere. Let's just make this rule well-looking.
1) White combo colours, as Zare said, cause bad kiai strobing on some skins. I'd not use "pure white" as definition, because #FFFFFE colour is, nominally, not "pure", but still does the same thing.
2) Pure black combo colours make hitobjects invisible with 100% dim (and default skin).