what style/2 for syntax coloring of literals in DCGs?

foo("hi").  turns "hi" a pleasant shade of blue in pceEmacs.

foo --> "hi".  turns the "hi" an unpleasant red.  This is particularly a nuisance since
it's the same syntax coloring as undefined stuff.

looking in prolog_colour I'm uncertain where this red's coming from, so I'm not sure
how to define emacs_prolog_colours:style/2

def_style(string,   [colour(navy_blue)]).

would, one would guess, apply, but apparently not, since obviously the color's not navy_blue

I've got to say, one of the nicest things about SWI-Prolog is the accessibility of most of this
sort of thing. Most of my projects, when they start up, set the environment just like I like it,
bring up the web page I'm working on, etc.