To add to Fencer's reply, in all the newer skins a file called material.txt is added to the skin folder along with the bmp when you save the skin from the game. When you open this file it will look something like this:
0.359,0.359,0.359,Ambient
0.759,0.759,0.759,Diffuse
0.000,0.000,0.000,Emissive
0.122,0.122,0.122,Specular
15.000,Power
The key line here is the one ending in specular. This controls how shiny the skin appears. The above file has a low figure and will give a matt appearance, fine for an OD skin. The one below has a much higher specularity and would look good on a bare metal skin.
0.302,0.302,0.302,Ambient
0.439,0.439,0.439,Diffuse
0.000,0.000,0.000,Emissive
0.756,0.715,0.650,Specular
8.200,Power
If you don't add a material file to a P-47 skin it will revert to whatever settings the default skin uses. There is a materials button in the skin viewer which lets you play around with these settings.