bakafox, the moon image can be found from your program directory Sims files.
TheSims(TM) 3\Game Data\Shared\Packages\ and it is the FullBuild0 package like simsample said. As you can see in my screen shot, the file path may differ if you used Origin to install your games.
I opened that up in S3PE and sorted the files to find the _IMG file with the tag of "moon".

simsample, I'll be happy to share my color ramps after more tweaking. As you can see in this moon photo, my basic colorization of the color ramp even produces a teal sky at night, and that's something I want to correct.

So night sky color, and horizon color, those are the two things I need to figure out.

ETA...you can download my moon DDS from here