The passive lighting is apparently set inside the source code rather than by script.
It did seem hardcoded in this case, thanks for the confirmation 4zb4. I can likely get around it for now with luminescent ground materials to fake daylight to a degree, but that obviously isn't a viable long term solution.
IMO sunlight colour doesn't need to change, so long as we assume there's only a single sun. A lot of the location aesthetic can already be handled via ground clutter, material choices and BackGroundType. Instead of touching sunlight colour, it might be better to instead implement a basic ambient colour filter. This'd allow for more flexibility in dungeon building in general instead of being limited to above-ground locations. A slight orange haze for caves in volcanoes, a pale yellow for a sweltering dusty desert, a bluish tinge for glazier caverns, a dark green touch for dense rainforests or so on. This is not to say I've mocked up the effect on a screenshot, so no guarantees I'm not just speaking out of my ass and it'd look like shit.