Guidance varies. Do we uppercase or lowercase the first word following the colon if it's a complete sentence? There is a big difference between the two brothers: one is an introvert and one is an extrovert. Thank you.
A colon does not complete a sentence, it separates phrases (in your example, two independent phrases). Ergo, no capital.
From Chicago's Q&A: Q. My grammar books say to cap the first word of what comes after a colon if what comes after the colon is a complete sentence. I noticed you didn’t do that. (“Check it out in printed books and magazines and newspapers: you probably won’t find any double periods after abbreviations.”) A. It’s fine to capitalize there, although Chicago style is to lowercase after a colon unless what follows consists of two or more complete sentences. Please see CMOS 6.64 for examples and exceptions.
My question would become why the colon and not a semicolon? My personal feeling as regards the use of the colon to separate a complete clause from another complete clause or fragment is to set of a list of items, a separate event or paradigm, or the rhetorical answer to an implied or actual question. A bit like a punctuation version of thus or ergo. In your original example: The idea of a rhetorical answer to an implied question is very present. Q) What's the difference? A) Introversion v. extroversion. But in the example given by Chicago's Q&A, I don't feel that same dynamic. I would have gone with a semicolon or a simple period. Curious.
Capitalization is allowed if the part following the colon is a complete sentence. I only learned about this last year. I saw another member on here capitalizing the word after the colon, and I was going to say something until I looked up the rule and found out that this is acceptable. However, this is a personal preference. Either way is fine. Also, I do agree with Wreybies about using either a period or semicolon. Although I could see someone using a colon in this example, it seems a bit archaic.