Q. CMOS gives a way to cite a book with two subtitles: by using a colon and then a semicolon between the three pieces of the title. What if the book I want to cite already has a colon printed between the first and second subtitles (no punctuation between title and first subtitle)? Is it okay to insert a colon between the title and first subtitle, then change the printed colon to a semicolon between the first and second subtitle?
A. When you say that there is no punctuation between the title and subtitle, it sounds as though you’re looking at a book cover or title page. Punctuation in a citation is not based on covers and title pages (“display type”), because there often isn’t any punctuation there. In display, the title is often set on its own line and given special treatment, like larger type or color, which conveys to the reader where the title stops and the subtitle begins.
Short answer: yes, in a citation you must insert punctuation to convey where the title ends and where the first subtitle ends. Check the Library of Congress information on the copyright page to confirm your impression. (The various subtitles will be separated by colons there; in your citation, you can change a colon between two subtitles to a semicolon.)