Unlike in the manga and most other media Seto Kaiba had green hair in this series.
His appearances in this series are based on his appearances in "The Cards With Teeth" and Death-T from the manga. Additional aspects, such as The Four Game Masters are also added and various events changed.
Kaiba is nearly identical to the manga version except he has green hair and yellow eyes in this series. The Toei's version of the anime run for the first time in April 1998, while the fourth volume of the manga - Kaiba's Revenge - was released September 4th in 1997. In that fourth volume, Kaiba is clearly depicted as having brown hair in one of the color page of the Chapter 27. The creators of the show may have known Kaiba had brown hair from the beginning. One possible explanation is that Green is the color of envy in western culture and english-speaking countries.
Death-T was a "theme park" created by Seto Kaiba in-order to kill Yugi Motou. He made this after he tied his first duel with Yugi Motou. Kaiba invited Yugi and his friends to the Death-T theme park. When Yugi arrived, he found out that Kaiba's men had kidnapped Yugi's grandfather, Solomon Muto. He then challenged Yugi's grandfather to a Duel. As a result, Kaiba won. He punished Yugi's Grandfather with a "penalty game", creating virtual holograms of 3 "Blue-Eyes White Dragons". Solomon received too much of a shock from the holograms, and thus lost consciousness.
Yugi Moto swore to avenge his grandfather. He had to battle his way up to the top levels (Death-T 4 and 5) where Kaiba was waiting along with his brother, Mokuba Kaiba. Yugi managed to get to the top level, but before dueling Kaiba, he had to defeat Mokuba in a game of Capsule Monster Chess. Yugi won and then started his duel with Seto Kaiba. He managed to defeat Kaiba with the five pieces of Exodia. As a Penalty Game for Kaiba, Yugi shattered his evil soul using "Mind Crush".