If a group of words following a colon could stand on its own as a complete sentence, capitalize the first word.
Clinical trials called into question the safety profile of the drug: A high percentage of participants reported hypertension and kidney problems.
Preferences vary among academic disciplines. See MLA-5, APA-5, and CMS-5.
Always use lowercase for a list or an appositive that follows a colon (see P3-d).
Students were divided into two groups: residents and commuters.