As we don't know the details of the story, the main character is presumably the man in the suit.
Obviously, the main character is the man in the blue suit.
Based on just the title, it's likely the man in the black suit is the main character.
I'm not sure as I haven't read this graphic novel specifically. But usually, the main character could be the man in the brown suit himself.
It could be a young boy named Tom. Tom is a curious and energetic kid who loves dinosaurs. One day, he gets a dinosaur suit and his whole world changes. He starts to see himself as a real dinosaur and acts accordingly. His family and friends then become part of his dinosaur - themed adventures.
Since I haven't read the 'the boy in the black suit novel', I can only make general speculations. The boy in the black suit might be the main focus. He could be a mysterious figure. His black suit might give him an air of formality or mystery. There could be other characters in the story who are influenced by him or are trying to figure out his secrets. Maybe the story is about his adventures, his relationships, or a problem he is trying to solve.
The main character is the man in the gray flannel suit himself, of course. But there are also likely other characters like his family members, colleagues, and bosses.
As we don't know the details of 'the man story', the main character is simply 'the man' in question.
There's no way to be certain without more information about the 'black suit story'. However, it might be a detective in a black suit, if it's a mystery story. Or it could be a businessperson who wears a black suit and is facing some challenges in their corporate world.
The main characters in the 'black suit story' are really up in the air without more context. It could be a young person who inherits a black suit from a relative and their journey to discover the secrets behind it. Or perhaps it's a group of friends, one of whom always wears a black suit, and their adventures together are the focus of the story. There might also be an antagonist in a black suit, who opposes the protagonist throughout the story.
I'm not sure which novel 'the man in the brown suit novel' is specifically. There could be many novels with such a title or a similar description. So, unfortunately, I can't tell you the author.