With lightning themed techniques, Zenitsu’s hair has come to represent the Thunder Breathing he’s known in the series for. But how did Zenitsu’s hair turn yellow?
Zenitsu’s hair turned yellow because he was struck by lightning. He was hiding in a tree during a training session with the Thunder Hashira, Jigoro Kuwajima, when lightning struck the tree and changed Zenitsu’s hair from black to yellow.
He kept the hair color for the rest of the series despite it not being his natural color.
Plot Reason For Zenitsu’s Hair Turning Yellow
Chapter #33 presented us with flashbacks of Zenitsu’s training with his grandfather, former Thunder Hashira, Jigoro Kuwajima. Zenitsu felt a lot of frustration not being able to live up to his grandfather’s expectations. He could not handle demon slayer training and could only perform a single breathing technique. This takes place before Zenitsu’s final selection entrance exam.
“When I was being financially supported by the woman who eloped with me and was riddled with debt, you saved me! Or, I might have just wanted to be brought up as a swordsman.”
Zenitsu speaking to the Thunder Hashira | Chapter #33
Zenitsu expressed that he wants to live up to his grandfather’s expectations, but can’t. He said he feels tremendous regret over his performance, and he feels insecure that the former Hashira may be hiding Zenitsu out of some embarrassment.
His grandfather tries to calm Zenitsu down by saying he does have talent, but in that moment lightning strikes the tree that he was clinging on to.
“What an unpleasant life… I got struck by lightning and my hair color changed. I was thankful for just being alive.”
Zenitsu’s inner thoughts after the lightning strike | Chapter #33
From that moment onward, Zenitsu’s hair has been light. Considering that he was being trained by the Thunder Hashira, it can be interpreted as some type of message from a higher power that Zenitsu was struck by lightning.
So that’s the answer. Zenitsu’s hair turned yellow because he was struck by lightning while training with the Thunder Hashira. His hair color was originally black. It would make sense that due to his breathing techniques and master’s expertise that the lightning strike represented some type of awakening for Zenitsu’s breathing techniques.