All I can say is that those with white-colored hair are the strongest, while those with black-colored hair are the weakest. White is the presence of all colors, which makes it the strongest. Black, on the other hand, is the lack of color, so it's very weak. This is why Youki left, because he didn't want to accidentally kill everyone with his overwhelming power. Youmu will do the same when she realizes her full potential, and so she will never, ever make an appearance in the series again.
What if you see a doll dressed in red and black, a purple umbrella, a Noh mask, a lute, a koto, and a taiko drum at a garbage dump?