This game is so hard to play without turning all of the lights off. Like, I thought I might try to use my addiction to the game to get myself to wake up at a reasonable hour, so I get up, turn on some lights (don't want it to be dark, as I want to wake up), but then I find I can barely see what I'm doing. It's so dark.
I took some screen captures and applied a brightness adjustment:
http://imgur.com/a/2ZeJG
I applied the same adjustment to day and night images, and it looks fine on both. Day looks like day, night looks like night, dead stuff still looks dead, live stuff finally doesn't look dead too. So this wouldn't be hard to do. Basically the only thing that oversaturates is the user interface, since it's not so dark.
I should probably expand on the night brightness since a friend wasn't convinced by the image... Basically, it's not that dark in the dark. Like, I was watching an episode of NCIS where they went into a dark house with flashlights and spent ten minutes sneaking around trying to find the bad guy without him shooting them, and the whole scene was nearly pitch black, only lit up occasionally by a flashlight. Clearly that scene was created by someone who's never used a flashlight in a house at night. Turn on the flashlight and you can see everything, not just what you point the flashlight at. Light reflects off of everything and hits everything else, not just what the flashlight is pointed at. Indeed, what works best is to point the light at the ceiling, so that you have equal lighting all around rather than bright lighting where the light is pointed and dark lighting elsewhere.
Now outside, that doesn't work the same obviously, since the light just goes off forever rather than bouncing off of nearby walls. However, outside also often has this thing called known as the "moon," and it reflects a lot of sunlight to the earth, such that you can easily walk around outside in a rural area with no flashlight and see literally everything. Now people who live in a city may not understand this, but that's because a bright streetlight causes your iris to close somewhat, and thus when you look where the light doesn't hit, it looks quite dark. However, if the streetlights weren't there, you'd be able to see everything under moonlight. ...and of course, when you do place lights that don't cast shadows, you can see quite well.
I'm sure someone will argue that moonlight isn't always present, but then I'd just argue that it also isn't never present. Indeed, it's probably easier to argue that it should be always present. Google tells me that there are 146 moons in our solar system. That's an average of 18 per planet. What are the odds that, on the particular planet where Factorio takes place, that at any given moment, there isn't at least one moon in the night sky providing some light?
So there's no reason that a game (or a T.V. show) has to be black in order to convey that it is night. Just cut the lighting a little, introduce more shadows, and reduce the color saturation (at night you see in black and white), and it'll look like night despite still being bright enough to see. Indeed, it would probably be amusing if it was necessary to use your flashlight or to place lights in order to tell what color your machines and inserters and belts were at night.
On a separate but related note, I think the blacks are too black. The thing about black is, in a well-lit scene (like daylight), even black is a bright color. The only time you see true blackness (like RGB color #000000) is when you're looking into a deep hole, so deep that there just isn't any light down there. Every other black object is reflecting light back to your eyes and the only reason you see it as black is because your brain knows it is black. So there probably shouldn't be any near-zero color values in the daytime.
To show what I mean, here's another edited screenshot, where I remapped the colors such that there are no color values below 16 (out of 256):
https://imgur.com/a/iTdyx
For another example of how true black is unnatural, look at this image:
https://en.wikipedia.org/wiki/Vantablack It shows a material made from nanotubes such that any light entering is reflected on the tubes so many times that it's virtually all absorbed. So it is true black. ...and it looks like someone broke reality. For normal things, no matter how black they are, you'd be able to tell that they were wrinkled up because they'd still be reflecting some light, but you can't tell the foil is wrinkled because there's no light coming off of it.