I decided to write a little "How the _NRM.png and _NML.png files do work" text because I just had to deal with it myself again. Over time I forget all of this and then I have to look it up. So why not doing it publicly.
I just got the olisim Lockheed L18 and like so many planes that have been designed for XL10 it is "fully updated to XP11" - and the liveries still look awful because nobody took a little care of the normal maps. Often old planes get a "one normal map fits all" file and that´s it.
I will upload the liveries I reworked when they are ready.

Those two files (_NRM.png or _NML.png, normally just one of them is present) make the paint shine or be matte or even look like chrome. I refer to them as "normal map" from this point.
Let´s pretend we have a completely white aircraft.
- if the normal map is fully transparent then the paint will be matte white like military paint.
- if the normal map is fully brownish then the paint will look glossy like a brand new piano.
- if the normal map is fully purplish then the paint is chrome.

Of course that can be graded and mixed. So if the normal map is half transparent purplish then it won´t look like chrome but more like brushed aluminum.
And if it is a chrome plane with writing on it then you will find the writing in the normal maps as transparent or brownish to make them somewhere between matte and glossy.

With a purplish normal map a yellow plane would look like gold. Red, blue, green and so on would look like anodized aluminum.

But - there is always a catch! There has to be a line set in the concerning .obj files that says NORMAL_METALNESS. All the new planes have it set of course so you don´t have to bother anyway but e.g. in the default B58 it is not set by default. You may stumble over that from time to time.
And if you have old liveries they might be effected by that and look different than before.
Normally (there are always exceptions) you will find the .obj files in ...\X-Plane 11\Aircraft\YourFolderName\YourPlaneName\objects

There is an X-Plane article about all that (I don´t understand even half of it): https://developer.x-plane.com/article/x-...material-model/