Perhaps it's a genetic thing that gets triggered later in life. In some fanfictions, Naruto's family history could be rewritten or expanded. There might be some hidden genetic trait from an ancestor that suddenly surfaces, causing his hair to turn red. This could also be linked to a new power or ability that he has to learn to control as his appearance changes.
One possible reason could be some sort of new chakra transformation. Maybe he taps into a different power source that affects his physical appearance, including his hair color. For example, if he were to inherit a special kekkei genkai from an unknown lineage that has the side - effect of changing hair color when activated.
One possible reason could be extreme stress or trauma. In some fanfictions, Naruto might go through a really tough battle or face a great loss that drains him both physically and mentally. This could potentially cause his hair to turn white, similar to how people in real life can get premature grey hair from high stress.
Maybe he just wants a new look. Naruto is always known for his wild and adventurous spirit, so dyeing his hair red could be a way to show his rebellious side or simply to stand out more.
One possible storyline could be that Naruto meets a red - haired character who has a special ability related to fire. They team up on a mission and Naruto gradually develops feelings for this character because of their courage and unique skills. The red - haired character might also be from a different village, which adds an element of intrigue to their relationship.
One possible plot could be that Naruto is born with red hair due to some strange genetic mutation in the Uzumaki clan. He then somehow gains the Rinnegan, perhaps through a powerful encounter or a hidden inheritance. With these new features, he might face challenges from both within the village who are suspicious of his new powers, and from outside threats who see him as a greater threat or a valuable asset. He could embark on a journey to master these powers and prove himself a worthy protector of the village, all while facing off against new and old enemies.
The plot could involve Naruto being part of a secret clan that has red hair as a symbol. He discovers this fact when he accidentally activates a hidden jutsu that only members of this clan can use. This jutsu might be super powerful but also dangerous. Naruto then has to learn about his clan's history, which is filled with both great deeds and dark secrets. He must decide whether to embrace this new part of his identity and use the power for good or reject it to stay true to his old self.
There's also the possibility that it's a transformed version of a character we already know. For example, Sakura could be magically or experimentally given red hair, and Naruto, seeing this new side of her, starts to develop romantic feelings. This could lead to a whole new dynamic in their relationships with other characters in the Naruto universe.
Perhaps in the fanfic, Naruto is trying to fit into a different group or culture. His original hair style might make him stand out in a negative way in that context, so he changes it to be more accepted. Or it could be part of a bet or a challenge. For instance, he loses a bet with his friends and as a consequence, he has to change his hair style.
One possibility is that it gives him a more mysterious and alluring look. In fanfiction, authors often use physical changes to symbolize a character's inner transformation or special powers. Black hair and red eyes might signify a new, more powerful Naruto, perhaps with a connection to some ancient or dark power source.
Maybe the red hair is a result of a genetic experiment or a curse that is linked to the Rinnegan. In the fanfic, it could be explained that an ancient ninja tried to combine the power of the Rinnegan with a powerful fire - type jutsu. The experiment went wrong in a way that gave the character red hair and also altered the Rinnegan's abilities. For example, the Rinnegan might be able to absorb and redirect fire - based attacks more effectively because of this connection.