Suggested techniques for converting a daytime map to a nighttime map?

Hi everyone - this is more of an artsy-craftsy question.   I use a lot of encounter maps from the excellent Gabriel Pickard  in the game I run.  His work is fantastic.   My dilemma is that the characters are going to be revisiting a location they've been in before, but while it was bright and sunny the first time, this time they will be returning in the dead of night.  Anybody have a good idea for how to convert a day time map to a darker color scheme so that it looks like nighttime? Any thoughts appreciated.  

Create a dark blue, but transparent image, and then stretch it to be over the full map, or at least the outside part of the map, if the maps have indoor locations as well. If you use DL, turn off "Global Illumination", and have torches/light illuminate only the right spots. You can have the lights have a yellow or orange aura to really sell the difference in illumination. If the blueish filter and DL stuff only kinda sells the night time, maybe use some ambient nighttime sounds like cicadas doing thier thing or having the occasional owl sound in the distance.

A simple effect I've used in the past is to just add a dark blue tint to the map 'token', if you have indoor sections then it doesn't work as well and Andreas' idea is a better, flexible, solution.

Thanks guys - that's good stuff.  Easy to implement and I've already given them both a try.  Thanks for taking the time to answer.  I appreciate it. 

I use Ziechael's method, go to map layer, double-click the main background Map to bring up its Token config panel, set the TINT to either Dark Blue, or Black, it will darken the map!