Tulamid's Colour Overlay Effect would make day/night systems a LOT easier, but... There's two main things to figure out.
1. The exact colourations and intensities needed for certain stages of the day, such as dawn, sunset and night (the third one is already provided), and how they'd transition over the cycle
2. Light sources "erasing" the effect at night.
Obviously, the erase effect doesn't work well with Light objects, so... I'm kinda clueless on how to get the second working. Yeah.
Current test cap for the night colouring, and a light source.