Another theme could be survival. In such a world, characters would have to struggle to stay alive. They might face shortages of food, water, and shelter, and have to constantly fight off threats from other desperate people or dangerous creatures.
Survival. Given that the world is not fit to live in, characters will be forced to focus on basic survival needs such as food, water, and shelter.
It could be about a post - apocalyptic world where the environment has been so severely damaged that basic survival is a struggle. For example, there might be no clean water or breathable air, and the few remaining humans are constantly fighting over scarce resources.
The challenges they face in this unfit world can shape their values. A character might initially value material possessions but, after seeing how meaningless they are in a world on the verge of collapse, come to value things like human connection and community. This transformation can be a key part of their development.
In many fanfics, a world might be considered unlivable if it's full of constant war, extreme poverty, or widespread injustice.
One possible theme could be cultural diversity. Since it's about the 'colors of the world', it might explore different cultures and how they interact. Another theme could be unity in diversity, showing that despite our differences, we are all part of one world.
One possible theme could be extreme despair. In the Harry Potter series, Harry faces many challenges, but in this fanfiction, perhaps he's pushed to a point where he loses all hope. Maybe he's faced with a loss that he can't bear, like the death of all his friends or the destruction of Hogwarts beyond repair.
One possibility could be that after facing too many losses, like the deaths of his friends in the fight against Voldemort. For example, if Hermione and Ron were killed right in front of him during a final battle. Harry might feel that all his efforts were in vain and lose the will to live. Another reason could be that he discovers some dark secrets about the wizarding world that shatter his beliefs. Maybe he finds out that Dumbledore had some truly evil intentions all along and that everything he fought for was based on a lie.
Perhaps he is betrayed by those he trusts. In the original series, there were instances of betrayal, and if in the fanfiction this is amplified, for example, if the whole wizarding community turns against him unjustly. He might feel so alone and hopeless that he loses his will to live. Also, if he discovers some dark secrets about his past that make him question his own identity and worth, it could lead to this state.
It could be due to an allergic reaction or maybe the character has caught a cold.
In Percy Jackson fanfiction, Goku could be a mysterious outsider who comes to help during a great war against the Titans or some other major threat. His Saiyan fighting spirit and his various energy attacks, like the Kamehameha, could be seen as a new form of magic in that world. He might not understand all the Greek mythology stuff at first, but his pure heart and desire to do good would make him a valuable ally. For example, he could team up with Percy to fight off a horde of monsters. His super speed and strength would give them an edge in battles.