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.
It might be related to a new form of training. Naruto is always looking for ways to become stronger. If he were to train in a very extreme and unorthodox method, like using some kind of rare natural energy that has a mutagenic effect on his body, it could turn his hair red. This new form of energy could be changing his body at a cellular level, and hair color is just one of the visible changes.
It could be due to a curse. In the Naruto universe, there are many dark curses. Perhaps an enemy has placed a curse on Naruto that not only affects his strength but also his appearance. The red hair could be a symbol of this curse, showing that he is marked in some way.
One possible reason could be extreme stress or trauma. In the Naruto universe, he has faced many difficult situations. If a fanfic writer wants to show the toll it takes on him, white hair could symbolize that. For example, if he loses someone very close to him like Sasuke in a more permanent way, it might be so devastating that it causes this physical change.
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.
In some fanfics, it could be related to a prophecy. His family might believe that Naruto is going to bring about a great disaster according to a misinterpreted prophecy. So, they turn on him in an attempt to prevent the prophecy from coming true, even though they are wrong in their understanding.
His confidence could be affected in different ways. On one hand, the new look might make him feel more unique and powerful, giving him a boost in confidence. On the other hand, if people react negatively to his red hair, he could start to doubt himself. This internal struggle could be a major part of his character development, as he tries to figure out who he is with this new appearance.
It could be a sign of a connection to an ancient or powerful magic. In some fanfics, a character's hair turning white might indicate that they are now linked to a long - forgotten magic that has started to manifest within them. This magic could be so strong that it physically changes their appearance. For instance, they might have come into contact with an artifact that contains this magic and it starts to change them, starting with their hair.
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 storyline could be that Naruto is born with red hair due to some sort of genetic mutation or a hidden family trait. This could lead to him being ostracized in the village at first as he looks different from the norm. But then he uses this unique feature to his advantage, perhaps by standing out more in combat or in his ninja training, as his red hair makes him more recognizable and intimidating.
It could be due to a genetic mutation in the Naruto universe. In some fanfictions, there might be a hidden genetic factor in his family line that gets activated. For example, perhaps Minato or Kushina had a recessive gene for white hair that Naruto inherits and it suddenly shows up under certain circumstances, like extreme stress or a coming - of - age moment in his ninja journey.
One reason could be that Naruto wants to give a fresh take on the story. Maybe he feels the original fanfic doesn't do justice to the characters or the plot. Another reason might be to attract more readers. If the fanfic has become a bit stale or predictable, changing it can bring in new fans.
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.