Subtitle Template

I was trying to organize my library and wanted a column for book subtitles (there isn't one built in already). How do I use custom template functions in the custom column (built from other columns) to take everything after the last colon and space as the output? I have tried several different things to no avail.
For example, the title column "Atomic Habits: Tiny Changes, Remarkable Results: An Easy & Proven Way to Build Good Habits & Break Bad Ones" would feed into a separate subtitle column that just says "An Easy & Proven Way to Build Good Habits & Break Bad Ones".

Thanks in advance for any help!