It's a bit more complicated because you have to create seperate terrain files (.TER files) for each sky your loading. So I wouldn't really advise it unless your only doing two random skies. Any more than that, and your file size will rocket. I can send you details if you would still like to do it though.
Edit: Here you go:
If you only want to darken one terrain for night you just have to make one extra sky folder, and terrain, so you can load two skydomes in one sky folder, and the night/sky in the other sky folder.
Set up the random skies as normal, but make two seperate sky folders, one for day/morning etc, and a seperate one for the night sky. Then copy the ground textures from the world1 one folder, but rename them with the same number of letters/spaces, and place them in the second night sky folder, in a msh folder.
Then copy your terrain file ABC.TER, or whatever your three letter map name is called, and rename the copy TWO.TER, or whatever you want, and leave it in the world1 folder.
Then hex-edit the renamed terrain file to call for the new names of the textures in the sky2/msh folder. Then darken the renamed textures in Gimp to what you want.
Then put this in your sky2.req:
...and this in your sky.req to load the original terrain file along with the day sky. ABC, or whatever your map name is called.
Then add a terrain ABC.req, and TWO.req in each seperate sky/req folders along with there respective sky req files, in your case day.req, morning.reg, and night.req.
Lastly delete both the LightName and TerrainName from the ABC.wld file:
Munge each sky folder seperately both with folders called sky but with seperate .req names sky.req for day/morning, and sky2.req for night, and so on.
Lua code would then look like this:
Edit2: If you want to edit your world in ze afterwards you have to put the terrain name back in the .wld file i.e: