Ah yes, Mask tiles. A bit more annoying to do than surface tiles, but they make the whole thing look a whole lot better once water and night lights are added!
So, how do we do them? Well, it is very easy.
First, I'd start by copying all of your folders in your Earth/Surf folder and paste them in Earth/Mask.

Then, you open your tiles (I'm using a single tile from Quebec as an example) in Photoshop or whichever image editor you use. And then delete all the water, either with a roto tool or the magic wand thing.
Preferably, you would use an alpha channel for this step, painting the water black (and so transparent) and the land white (opaque) and be done with it. However, in my personal case, my photoshop doesn't seem to want to save alpha channels in DDS files, so I have to resort to an alternative and slightly longer method to mask the water.

Once your water is deleted (or masked in an alpha channel), you then have to create the night tiles, otherwise the tile would look lit up in the middle of the night! One method you could do this is paint the whole thing black. However, without night lights (aka street lights), it wouldn't look very good in game. But, it is the easiest method and would ensure you have masked water.
The second option is a lot more time consuming. It however gives the best results. It is to hand-draw the street lights yourselves! The way to do it is to have your day tiles as one layer and colour corrected to look yellowish or white, and then put a black layer on top. Then, use the eraser tool to create dots in the black layer, making the bright layer visible under, giving the impression of lights lighting up the roads.

In game, it look very good! However, it can be time consuming on large spaces so I would recommend only doing it for airports and stuff like that.

The third alternative is the best option in my opinion, at least when it's time to do night tiles for large surfaces like an entire city. It is to simply use the Brightness/Contrast tool on the tiles! In legacy mode, with brightness at -100 and Contrast between 40 and 70 (depending on the result), you can get some passable street lights!
However, it only work best when you're doing it for cities, since it highlight the roads and the buildings. For bases in the countryside or the middle of nowhere, it doesn't work as well as it tend to highlight things that have no place being lit up (like beaches, farm lands and others). In these cases, you might need to pass over with a black brush and paint them out before saving the tiles.

And once you're done, you can save the image! If you used an alpha layer, then your tile should be good to go. However, if you simply deleted the water like I did, then there is one last step you need to do (unless there is no water in the tile you're editing, then it is not required).
But why is there another required step, you might be asking. Well, if you try to put your tiles in Orbiter and load them, you will see something strange: the water is masked in the day like it should be. But, in the night, it is completely fluorescent!

So, what is happening? Well, if you open the tile in photoshop again (make sure to close it first), then you will understand why. When the pop-up "Load Options" appear, make sure to click on "Load Transparency as Alpha channel". Then click okay.

You will get this surprise! The water is all white! The way to fix it is easy. Take the bucket and fill it black!


And then save again!
And then you should be done. You should now have night tiles in your base!

(And yes, if you're perspective, you will have seen that we re-opened the tile with the transparency as an alpha layer. So why could I save it an alpha layer now but not before? No idea. I think it's because before I created my own alpha channel while in this case photoshop itself did. I guess photoshop just doesn't care about my alpha channels and ignore them.)