@hob-B1T If you start with all your dynamic range (bits), or better yet exr, then you have somewhere to go in post. Night time is even more important to use your full dynamic range as shadow gradients are very important. Most films that have night time scenes are very often shot in day light or brighter light, then darkened in post, digital is getting more sensitive but the technique still used. Lighting is all about contrast ratios. A good way to learn is to take a still from a movie you like and emulate the lighting contrast and ratios. Happy learning. 
usually as it becomes night the red and blue saturations increas… but in @@jaypjohnson’s it appears to just have higher contrast. i suggest taking a hint from him, but re working it a tad. try using gimp to darken it, then increas contrast so colort are about where they where when you started, but the dark parts ar darker. then increase blue and red saturations.
the sky is a little off, but if it where i who where rendering this i would color it from the start. here is what i did in gimp. i use a layer filter wit ha gradient from white to pink to yellow to blue, to create the setting reflections. i then darken the scene by removing all colors but black green and blue from a duplicate layer, then do a grain extract to darken the dark parts of the film. a little touch up here and there and here is my result.
PS i know i over did it on the blue in the lower right… should have used a more saturated blue
Thanks to both of you @jaypjohnson and @ThorntonStrolia. I currently play in Gimp/Paint Shop Pro with Brightness/Contrast/RGB curves as it is faster than do the same in the node editor in blender (I’ll aways force to do as much as possible in blender, as I assume it works on raw values (32 bit ?)).
@ThorntonStrolia: Your image looks really good, but how can I accomplish something like this from rendering ? Should I add coloured lights sources ? I’m a bit conflicted between always try to light a scene in a “natural way” and using fake lights to create a good looking effect (and besides all, a have absolutely no experience on this sector).
Are the any good tutorials which give detailed information about these steps ? There are endless tutorials about modelling and texturing, some about three point lighting and how to apply environment lighting in cycles. But I haven’t found much information about how to finalize a rendered scene and what can be reached in the post proc steps. (Hopefully you understand what I try to say, its hard to describe and english is not my natural language).