The style guide mentions that colons should be used to separate subtitles, except if there is an “alternative dividing punctuation” mark, e.g. an en-dash.
Does this apply only if said “alternative dividing punctation mark” is used on the front cover?
I have a couple of releases where the front cover does not have such a punctuation mark but the spine does (usually an em-dash or an interpunct).
Personally, I’d use what’s on the spine then but what do the experts say?