This is interesting. Grammarly says:
In British English, the first letter after the colon is only capitalized if it’s a proper noun or an acronym; in American English, the convention depends on the format but it’s frequently capitalized. If your writing must follow a specific format, check the rules for that format before capitalizing (if in doubt, it might be safer to use the lower case).
This rule applies only if the clause following the colon is independent (a complete sentence). If the colon is used for a list, don’t capitalize the words following it.
I had never realised there was a US/British difference about capitals-after-colons. Upon reflection I realise that frequently when typing, I find myself capping the first letter after a colon, then examining the clause following the colon and uncapping if it does not satisfy the "British" rule cited by Grammarly.
I must say that I think the British rule seems more logical (I would, wouldn't I?), but I am not brave enough to call the alleged American practice an error.
The University Of Bristol Faculty Of Arts (UK) says:
There is some debate about whether the clause following the colon should begin with a capital letter or not. If the colon precedes a formal quote, you should begin the language of that quote with a capital letter. If the explanation that follows the colon contains more than one sentence, you should use a capital letter. In other cases, some guides simply advise consistency, others advise that a capital should always be used.
Capital Community College (Hartford CT USA) says:
There is some disagreement among writing reference manuals about when you should capitalize an independent clause following a colon. Most of the manuals advise that when you have more than one sentence in your explanation or when your sentence(s) is a formal quotation, a capital is a good idea. The NYPL Writer's Guide urges consistency within a document; the Chicago Manual of Style says you may begin an independent clause with a lowercase letter unless it's one of those two things (a quotation or more than one sentence). The APA Publication Manual is the most extreme: it advises us to always capitalize an independent clause following a colon. The advice given above is consistent with the Gregg Reference Manual.
We also use a colon after a salutation in a business letter . . .
Dear Senator Dodd:
It has come to our attention that . . . . .
. . . and when we designate the speaker within a play or in court testimony:
BIFF: He had the wrong dreams. All, all, wrong.
HAPPY (almost ready to fight Biff): Don't say that!
BIFF: He never knew who he was.
We don't start business letters like that in Britain - it's:
Dear Mr Bloggs
Thank you for your letter...