I watched something that said that kids hair stripes combines with one another or something if so when and how

When he starts to awaken as a true Shinigami/Death God and when he starts to understand his purpose as a Shinigami is when The Lines of Sanzu start to connect.
In the manga, at least. I don't think they explain it in the anime.
And then all three connect and he moves on as the new Shinigami of the world. The end.