[Den-O]

No one ever thought the show would be this popular. The production crew have said as much. It was just a 1 season filler thing to bridge the gap til Reconquista, but it was so immensely popular that it spawned a sequel.

So likely the in story explanation will be he died his hair to be more like Sei who he probably looks up to like a big brother/idol. That or China had another little brother we never saw