There are several options for a night version. Depends what you prefer. I guess building a complete model therefore might be the worst one, since it doubles the data size.
1. You can use textranslate to shift the texture to a night-version. Then you you have normally to illuminate the model via xml at night. Pros: Just one model with no additions. Cons: it doubles your texture size.
Used for exaple here:
https://scenery.flightgear.org/app.php? ... ew&id=15702. You can add parts to the model that are illuminated in the model that would be selected at night. I have done this for example in this model:
https://scenery.flightgear.org/app.php? ... ew&id=3289For the night I only add some windows that become selected when it gets dark. The windows are just rects slightly in front of the building. So kind of fake, but if it's dark and you didn't go rather, rather close to the building you can't tell.
3. You can place SharedObjects via xml at your model, in this case probably lights, that become selected at night. I have done this for the red warning-lights in the model.
Pros: Your model can be smaller and less complex, with no redundant elements. Cons: Depends on that the SharedObject is available.
Using Rembrandt, Canvas or ALS there are even more possibilities, but I like to keep it simple and this stuffs dosen't work anyway in my old FG version here.