Using a colon is an excellent way to tack a list, dependent clause or other relevant information onto a sentence. Yet if you don’t use your colon correctly, your editor (and potentially your reader!) is going to notice. First off, remember that a colon can only be added to a sentence that’s already complete—in other words, if you couldn’t end the sentence right there with a period, you don’t get to throw on a colon and more information. But once you’ve got that straight, a new question arises: should the word following the colon be capitalized or not?
As it turns out, it depends. In American English, the old-school rule is to capitalize when the colon is followed by another complete sentence. If the colon is followed by a list, sentence fragment or other dependent clause, no capitalization is required.
These days, many editors—me, for instance—choose to maintain consistency with all post-colon phrases: going with lower case in all instances, except when the first word is capitalized for some other reason, such as a proper noun or an acronym.
In the end, it’s ultimately your call. Whatever you decide, my best advice is to be consistent. If you capitalize a dependent clause on page three, it just looks weird to go with lower case on page six. So make a choice and then stick to it. Know your style and claim it!