Okay, Zanka thinks August is way too loud, but he can't help being jealous of his family relationship with Eishia. One day, August throws his arm around Zanka's shoulder and tells him he could be his older brother too. Zanka calls him an idiot, but he doesn't push his arm off.
I mentioned it before, but I think we should wait for characters in the story to mention it before treating "Rudo hair is getting darker" as a fact.
There's a ton of panel in the manga where Rudo hair looks as dark as it does in the post-Mymo battle.