But wait - what is additive lighting? Additive lighting is the basic equation X-Plane uses for "lit" stuff. Basically it says that the _LIT texture is added to the daytime texture to simulate the effect of an object that reflects sun light and emits its own light. Here's a more detailed description.
It used to be that the _LIT textures for instrument overlays were used instead of the daytime ones at night. Generic instruments introduced a new lighting mode, "back-lit", where the LIT texture is added to the daytime texture. This lets you make an instrument that has a light behind it to illuminate the markings at night.
In particular, when an instrument is back-lit, the amount of LIT texture added in is a function of the instrument light levels (the pilot can turn this up and down) while the amount of day time texture is a function of the sun and spot light shining on the area.
Originally I did not want to extend the legacy instruments to support back-lit lighting. What finally made me change my mind was the amount of detail in some of the standard mechanical instruments. As an exercise, I converted the six pack and nav instruments of Max's default Cessna to generic instruments.
The conversion required a lot of new (and very weird) datarefs, some esoteric extensions to the generic needle instrument, and it lacked some of the finesse of Austin's built-in instruments. All of these problems stem from one limitation: the animation action of generic instruments cannot be nested.
(Of course there is a practical consideration too - for an author with legacy instruments, rebuilding with generics takes time. Typically it took about 4-6 generics to model each built-in instrument in the six pack. The conversion only took me about eight hours, but I have access to the source code of the built-in instruments, a luxury authors would not be able to leverage.)
So in the long term, I am at least investigating the notion of nested animations and movements for generic instruments; I think that this would be the final flexibility needed to model just about anything with generic instruments.
But for the short term, you can back-light your built-in instruments; just set the lighting mode to "additive" and create the LIT textures.
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