Dragon Ball has always been a franchise marked by its characters growing progressively stronger by undergoing physical transformations. The most notable of these is the Super Saiyan transformation. The elevated state sees its Saiyan user emit a bright energy aura, and their hair stands up straight and takes on a golden hue. Franchise Akira Toriyama has revealed both the creative reason and in-universe reason for the Super Saiyan hair transformation.
With Dragon Ball Z, the first manga series to depict a Super Saiyan transformation, originally printed in black and white, Toriyama felt that Goku's hair shifting from his usual black to gold -- in this case, printed as white -- would help distinguish the transformation. The change would also spare Toriyama, who was working diligently to stay ahead of the series' anime adaptation, save time by not inking Goku's hair black during the transformation. This would carry over to subsequent Super Saiyan characters, including Trunks and Vegeta, as the series continued.
Years later, Toriyama explained the in-universe reason for the transformation in an interview. Toriyama revealed Saiyans all possess something he referred to as "S-Cells" in their blood -- not unlike Star Wars' revelation that Jedi and Sith possess a higher count of midichlorians to strengthen their connection to the Force. Anger amplifies the power of the S-Cells, resulting in the Super Saiyan transformation while the planetary conditions on Earth and mixed physiology with humans naturally increased the amount of S-Cells in Goku and Vegeta's children, leading to the characters achieving the Super Saiyan transformation far earlier than their respective fathers. Dragon Ball GT also included the non-canonical Super Saiyan 4, with the user's hair turning black again to represent them embracing their more bestial Saiyan nature.
The latest anime series, Dragon Ball Super, has further increased the Super Saiyan hair changes by introducing a new wave of transformations, blending the powers of a Super Saiyan with a god. Goku achieved Super Saiyan God initially by receiving power from five additional Saiyans. This made his hair bright red. This was a result of a mystical ritual and did not require all the contributing Saiyans to be Super Saiyans themselves, as evidenced by Pan. After unlocking Super Saiyan God, Goku and Vegeta learned to master the form and transform into a Super Saiyan while in it, with the resulting transformation aptly called Super Saiyan Blue due to the blending of divine and Super Saiyan power turning their hair cyan. And, at the end of Super, Goku mastered the Ultra Instinct transformation, embracing divine energy causing his hair to turn platinum like the angels that were seen using the form's combat boost earlier in the series.
The only other Super Saiyan hair color transformation to date belonged to Broly in his classic, non-canonical Dragon Ball Z incarnation. Broly normally possessed dark blue hair, but while restrained by a power inhibitor, Broly's hair turned into a pale green whenever he transformed into a Super Saiyan. Upon breaking free from the power inhibitor to attack Goku and the Z Fighters, Broly's hair transformed into the usual golden hue associated with Super Saiyans.
The Super Saiyans have quickly become one of the defining features within the Dragon Ball franchise, taking on new, more powerful forms as the franchise continues. Borne from a creative desire to visibly showcase had Goku had reached an entirely new level of combat power, the fan-favorite transformation has taken on many varied forms over the history of the franchise, which continues to thrill millions of fans worldwide.