It could also be related to a prophecy or a pre - determined fate in the fanfic's plot. There might be some kind of dark prophecy that says Kawaki has to kill Naruto to prevent a greater evil. Or perhaps Kawaki is being controlled by an external force that has a vendetta against Naruto and uses Kawaki as a tool to carry out the murder.
It depends on personal preference. Some might like the Boruto manga for its original storylines and character developments. Others might prefer Kawaki for its unique elements and style.
Kawaki makes his appearance in Boruto manga at a specific point, but I can't recall exactly. You might want to check the official manga sources for the precise chapter.
One common theme is friendship. Writers often explore how Himawari and Kawaki, who come from different backgrounds, form a bond of friendship. Another theme is growth. They both grow as characters, learning from each other.
The colorization in Boruto and Kawaki manga can be quite diverse. It depends on the artist's choice and the intended effect. Sometimes warm tones are used to convey emotions, and at other times, cooler tones set the atmosphere.