What are some of the contrasts between Mr Darcy and Mr Collins in Jane Austen's Pride and Prejudice, and how are they made interesting for the reader?
Your question (and the keyword association of "literary devices") corresponds to elementary characterization, which is a literary device of the category called "literary elements." Two literary devices useful to characterization, which are of the category called "literary technique," are the contrasting techniques of hyperbole and understatement. So the essential answer to your secondary question, "how are they made interesting?" is: Through detailed characterization.
Your first question asks you to work backward to the literary device element of characterization by finding details: "What are some contrasts?" This is where the literary device techniques of hyperbole and understatement are of particular usefulness.
Mr. Collins is characterized through the literary technique called "hyperbole," which is literary and/or rhetorical exaggeration.
Everything about Mr. Collins is exaggerated, which is the point Mr. Bennet makes very nicely for us when he reads Collins' letter aloud at the dinner table. Bear in mind that, while Austen is creating an hyperbolic character in Collins, one who embodies exaggeration, this is not to suggest that she intends him to be an unrealistic caricature of a personality, as he is portrayed in film versions. She does not. Austen has one of her most realistic characters introduce him by reading aloud his letter (Mr. Bennet) while another of her most realistic characters listens and comments (Elizabeth) upon the man behind the letter (Collins).
Aa an instance of Collins' hyperbolic, exaggerated characterization (if one is indeed needed for him, who rather speaks for himself), we can remind ourselves of his behavior at the Netherfield ball when he espies Mr. Darcy among the guests and insists upon introducing himself against all social rules of conduct.
"I have found out," said [Collins], "by a singular accident, that there is now in the room a near relation of my patroness [Lady de Bourgh]. ... I am most thankful that the discovery is made in time for me to pay my respects to him, which I am now going to do,..."
"You are not going to introduce yourself to Mr. Darcy?"
"Indeed I am. I shall intreat his pardon for not having done it earlier...."
Elizabeth tried hard to dissuade him from such a scheme; assuring him that Mr. Darcy would consider his addressing him without introduction as an impertinent freedom,... (Chapter 18)
His "singular accident" led to singular hyperbolic, exaggerated behavior that was well outside the bounds of propriety. Recall Darcy's reaction of surprised wonder at Mr. Collins' "attack": "[Darcy] whose astonishment at being so addressed was very evident. ... was eyeing him with unrestrained wonder, ...."
Contrast this to Darcy's understated behavior that is the antithesis of Collins' exaggerated deportment. Understatement is called "meiosis" and means that the words spoken or actions taken are subdued and minimally reflective of feelings, thoughts or events.
At the Meryton ball, Darcy, in his understated manner, stands aloof declining to converse or dance with people he is not formerly acquainted with. When visiting, he sits silently and observes the company about him, assessing their manners and counting their follies (with the Bennets and Collins, who can really blame him?) rather than interacting as his friend Bingley does. In true understated fashion, when impressed by Elizabeth's "fine eyes," he keeps his council to himself and only later teases Elizabeth about twisting things into what they do not mean for her amusement (she loves to laugh at folly: "Follies and nonsense, whims and inconsistencies").
"And your defect is to hate everybody." [Elizabeth said.]
"And yours," [Darcy] replied with a smile, "is willfully to misunderstand them."