In Chapter 16 of Robinson Crusoe, the narrator describes his attack on some unarmed islanders he calls "savages" and "cannibals." Having rescued their prisoners, he describes his position as master of the island and its population in comically grandiose terms:
My island was now peopled, and I thought myself very rich in subjects; and it was a merry reflection, which I frequently made, how like a king I looked. First of all, the whole country was my own property, so that I had an undoubted right of dominion. Secondly, my people were perfectly subjected—I was absolutely lord and lawgiver—they all owed their lives to me, and were ready to lay down their lives, if there had been occasion for it, for me. It was remarkable, too, I had but three subjects, and they were of three different religions—my man Friday was a Protestant, his father was a Pagan and a cannibal, and the Spaniard was a Papist. However, I allowed liberty of conscience throughout my dominions. But this is by the way.
The irony here is evident, but it is based on the scale, not the structure, of Crusoe's kingdom. He does not doubt the substance of any of the points he makes. He says that the island is his "own property," and is his by "right." The King of England (George I at the time) would have been careful not to say any such thing about England itself, as the execution of Charles I in the previous century was based on such claims. However, they did apply to the royal colonies, and Crusoe's assumption of ownership is a classically colonialist position. The statements that he is "lord and lawgiver" to all three of his subjects, and that it is for him to decree whether they are permitted to follow their religions, reflect similar assumptions.
At this point, Crusoe has been on the island for over twenty-six years. However, nothing that has happened has modified his initial view that he has a god-given right to regard the island as his own, and to treat everyone on it as a slave or a subject. This viewpoint remains consistent through all the vicissitudes of the narrative.
As others have noted, the chief way colonialism is exhibited in this novel is through Crusoe's treatment of Friday, the native whose life he saves.
Crusoe has unquestioned feelings of superiority over Friday and evaluates him by European norms, approving of his physical appearance, for example, because his features and darker coloring conform to a European notion of intelligence and attractiveness. Crusoe also fails to show an interest in Friday's culture or religion; he assumes Friday should and will conform to his culture as more "advanced." Like a typical colonialist, Crusoe gives this native a European name, converts him to Christianity, and makes him his slave (although his rule is benign and friendly).
Crusoe never interrogates whether it is right to treat Friday as an inferior; the idea that Friday might be an equal or have something to teach him never seems to even remotely cross his mind. (The 1960s retelling of the story by Michel Tournier, called Friday, is a fascinating take on the original, in which Crusoe does end up learning from Friday. You can read about it on eNotes.)
Crusoe also shows a colonialist mindset in his sense of ownership over the deserted island. He thinks of it as "his" and imagines himself as the lord of the domain. He sees nature around him as his to dominate and subdue for his own purposes. We as readers are meant to appreciate his hard work, perseverance, and talent in bending the land to his will. The novel never questions whether he has any right to take this land as his own or whether imposing a European farming system on the terrain is appropriate.
Some scholars argue that the entirety of British writer Daniel Defoe's adventure novel Robinson Crusoe is supported by colonialism, but this answer will focus on two specific elements of the book where colonialist attitudes are undeniable.
The transatlantic setting of the novel as well as Crusoe's status as a slave owner and a businessman involved in the slave trade can also be interpreted through a colonialist critical lens. At the time at which Defoe wrote Robinson Crusoe, the slave trade was underway and the British were expanding their business interests to countries all over the globe. Interestingly, in part 1 of the novel, Defoe himself is captured and made a slave, but he is able to escape his imprisonment.
Throughout part 3, when Robinson Crusoe develops a relationship with the native he calls Friday, Friday addresses Crusoe as "Master." At one point, Crusoe himself describes their relationship as that of a parent and a child. Friday converts to Christianity, under Crusoe's guidance, and he also learns to speak English. This relationship between subordinate and superior is indicative of an imperialistic attitude toward natives of lands colonized by Europeans (for the British in particular), and this attitude is emphasized by the pressure Crusoe places on Friday to change according to Crusoe's religious and linguistic preferences.
The theme of colonialism is everywhere in Robinson Crusoe. In fact, one might say that Crusoe himself represents colonialism completely. Crusoe immediately thinks of the island as his with no regard whatsoever for what came before him. He is a perfect representation of the ethnocentric English tendency to think of any indigenous group as savages and rationalize their exploitation.
Most of all, colonialism is painted clearly in the relationship between Crusoe and his servant, Friday. Crusoe names him Friday to remind him of the day that he saved Friday's life, always keeping him in Crusoe's debt. Upon being saved, Friday grabs at Crusoe's feet, and he puts his foot on Friday's head. There is no more fitting metaphor for the effect of colonialism on indigenous people.
To me, this book shows this theme in the ways that Robinson Crusoe interacts with "his" island and then later with Friday.
What Crusoe does on the island is classic colonialism. He takes the island and tries to remake it in ways that make it resemble England and in ways that make it more useful to him. He domesticates goats, he domesticates parrots. He plants non-native species of plants and he brings dogs and cats onto the island.
When he find Friday, Crusoe immediately makes him a servant and teaches him to say "master" before anything else. He calls himself the king of his island.
So this is a story in which Crusoe's actions symbolize what colonizers do when they come to "new" lands.