[JimmXinu]

The %(var)s substitution is a built-in feature of the INI file parser. It doesn't know anything about FFF metadata entries. (Honestly, I'd rather it didn't--we have to escape % to %% far more often than it's useful.)

I was all set to tell you it's not possible, when I realized, holy crap, it is! An ugly way I'm mildly ashamed of, but a way.

Way back when, I stuck output_css into the story metadata as a convenient (if cheesy) way to pass it to the writer code. And it's been that way ever since.

Which means output_css can be edited by replace_metadata.

Here's the basic version that works for me. Needed to add "Audiences" to use == matching.

[archiveofourown.org]

## fff_titlepage_color_replace can be something else, but it should 
## be unique within output_css and  ideally not conflict with regex
## The final FFFFFF line is just to make it a real color if nothing else matches.  

add_to_replace_metadata:
 output_css=>fff_titlepage_color_replace=>527200&&rating==General Audiences
 output_css=>fff_titlepage_color_replace=>A69905&&rating==Teen And Up Audiences
 output_css=>fff_titlepage_color_replace=>AD5600&&rating==Mature
 output_css=>fff_titlepage_color_replace=>980101&&rating==Explicit
 output_css=>fff_titlepage_color_replace=>1B1B1B&&rating==Not Rated
 output_css=>fff_titlepage_color_replace=>FFFFFF

add_to_output_css:
 body.fff_titlepage { background-color: #fff_titlepage_color_replace; }