Post-colonialism, as the name suggests, is a way to analyze the ways in which the oppressive power dynamics of colonialism continue even though formal colonialism is supposedly over and done with.
One major aspect of colonialism was the savage way in which the colonizers treated the colonized. They tended to treat them as disposable objects instead of as complete human beings. One way to talk about Flight as a post-colonial novel is by discussing Zits. The name itself draws attention to the inferiority that was attached to the colonized subject. The teen boy’s name reminds readers that the colonized could be thought of as blemishes or flaws that could be popped or killed with minimal hesitation.
You could also say that Zits’s orphan status makes Flight a post-colonial novel. It draws attention to the ways in which colonizers tended to destroy their subjects’ lineage or history. The violent separation leads to displacement. Zits is constantly displaced. He’s in various foster homes and time periods.
Additionally, Flight addresses the post-colonial debate on assimilation. Colonized subjects might lead safer lives if they don't resist the norms and customs imposed by the colonizers. However, by espousing the new culture, they might be abandoning or betraying their own way of life.
The fraught issue of assimilation and betrayal manifests in Flight when Zits turns into an FBI agent. In this part, he becomes a part of the system that oppressed the indigenous people. He ends up oppressing the indigenous people himself when he shoots an indigenous person.