Currently, securemodelines parses : as a delimiter (either for the end of the modeline or for the end of an option) and doesn't handle a : being used as part of the option's value. Vim also handles the escaping of : in a modeline different than escaping in a normal set command. vim:set isk+=\:: and vim:set isk+=\\:: both do the same thing, when the former is what's documented to work and the latter is what one would expect when escaping something in a normal set command.