This has been bothering me for years. Word’s grammar check does not object to either usage. I assume it’s a matter of style: the capitalization of the first letter of the first word after a colon, used in apposition, as in this sentence. I’ve always used two spaces after the colon, but don’t capitalize. On the other hand, I’ve seen capitalization used and only one space used. So, what do the different style manuals say?
I’ve never heard of a style in which two spaces were used after a colon, correctly or incorrectly.
The Chicago Manual of Style, 13th edition says:
The practice of two spaces after a colon, like two spaces after a period, is from the old typewriter days and is no longer used.
My trusty old Writer’s Hotline style book says
A capital letter following a colon emphasizes the elaboration that follows. A capital letter may be used when an independent clause or a single item follow a colon; it is not appropriate when a list of items follows a colon, unless a word that is ordinarily capitalized follows the colon.