Thanks a lot ryanrybot!
Depending on the time of day the spawning enemies will change too (and day/night cycles will be much slower than what I've shown til now). At night I want to have more futuristic and deadly enemies joining the battlefield. One of the other levels, a snowy version of the red square in Moscow, together with it's iconic church etc., f.e. will feature changing weather conditions and fighting the enemies in a snowstorm will provide an even greater challenge for the player.
The 6 point gradient map shader which I programmed for the color transitions currently only swaps between two color sets, depending on the time of the day. But as it is fed by a json file, I will add simply one more parameter with which you'll be able to have smooth color transitions for any amount of points in time, like a cool dawn, followed by a bright midday, a warm afternoon and a dark night.