OK, I finally succeeded in changing the analog clock to red. It wasn’t pretty and I don’t know if there is a better way. But here’s what I did, for the record:
- 
In modules/default/clock/faces, I copied face-010.svg to face-013.svg to create a new face. 
- 
I edited face-013.svg to modify the style to have a fill color: 
 <style>.cls-1{fill:#f00;}</style>
- 
Added the following to my custom.css: 
/* for the date display */
.nightclock .date.normal.medium {
    color: #f00;
}
/* for the hour hand */
.nightclock .clockHour {
    background: #f00;
}
/* for the minute hand */
.nightclock .clockMinute {
    background: #f00;
}
- Edit the config: section of the clock module in config.js.
- Added secondsColor: “#f00” to the config: section of the clock module in config.js to change the color of the second hand
- Change the analogFace config to the newly edited “face-013”.