docs.flightsimulator.com
The SDK has documentation on materials and I think it's important to apply material appropriately. Decals were created to make aircraft details sharper without adding large quantities of texture overhead. I believe decals have an emissive channel, but I believe that is intended for instruments to be controlled by the aircraft XML. Perhaps it also work like emissive textures, which overlay the albedo "automatically" at night conditions and emissive textures are what the SDK informs to use for scenery objects.
When I make an object like yours, I try to use photographic textures as much as possible. For me, it is easy to turn a day texture into a reasonable semblance of night, you can design it such that it works in congruence with any attached lights you add, which are good to use because they provide volumetric lighting well. You don't even necessarily need an illumination source for them.
For the row T hangars and the parking lot further back, I just created library objects that were rows of lights, only the light itself as a "wash" and I positioned those around the scene. Similarly along the back of the big square building, whereas the close side right corner has an emissive texture showing. A lot of the time in MSFS, you can make an adequate night scene with few night textures.
In your situation, for example, I would probably just use the day texture for the main building, model the sign as 3d, or at least a distinct polygon plane. For day it would be the same texture as the building and the night texture would be emissive, which is why I uploaded it so you could see how I build them and I'd also experiment with illuminating the back side of the sign to see if I could create a little light wash from it.
You could do similar with colored lights. Here's a couple of examples, the texture is transparent, you can see I forgot to put a "reverse" face inside the one light. The night texture is colored and we can also place a light and we can boost the emissive factor, I think the limit is 4 or 400%.
I have a philosophy to keep me in line that it is a flight sim, if I can't see it from the cockpit, we're probably ok with just a nod to eye candy.