In the Dragon Ball storyline, Son Goku's iconic golden long hair is a result of his transformation into a Super Saiyan. This transformation occurs when a Saiyan achieves an intense emotional state, such as anger or sadness, and unlocks a powerful energy within them. The transformation not only changes their hair color to a golden yellow but also increases their strength and speed dramatically. Goku first achieved this transformation during his battle with Frieza on Planet Namek and later mastered the form to become a Super Saiyan 2 and 3. For more inquiries, please check Cosplay Costume.