How did kaibas hair trun from green to brown?

Answer 1

It didn't really, apart from it being a design change. The mini-series with a green haired Kaiba is non-canon, apart from anything the main series references. The whole character of Seto Kaiba was redesigned.

Answer 2

Kaiba's hair color was inconsistently portrayed in different adaptations of the Yu-Gi-Oh series. In the original manga, his hair was depicted as brown. However, in the anime, his hair was initially colored green before being changed to brown in later seasons for aesthetic reasons.