The level of shinyness is related to the 'alpha' layer. This is a greyscale 'layer' in the bitmap that determines how reflective the tectures are. Black means 100% reflective, white is 0% reflective. Of course you can use any shade of grey in between too.
To use the alpha layer you must create your texture bitmap, and an alpha bitmap which will be combined using dxtbmp.
In the siplest terms, you create a bitmap that has all the areas marked on that will be reflective in an appropriate shade of grey (experiment to get it just right), and the rest white.
You then open the texture bitmap in dxtbmp, then import the "alpha channel" - select your alpha bitmap.
Then when you save the combined files as DXT3 or 32bit the alpha layer will be read by FS and shown as levels of reflectiveness.
couple of examples:
Dragon Repaints Typhoons, high reflectiveness from a dark alpha layer, where required, eg all the tail fins will have White on the alpha layer, the fuselages black.
My Hawk 200, low reflectiveness, as i used a light grey in the alpha layer over a grey texture file.
And i'm now going to copy that into the tutorials on my forum, as I havnt got that there yet
Hope thats been some help, and not too technical for a beginner. Feel free to PM or e-mail me if you want more help.