I have set up the Scania R to use normal skinning. 

I have set up the Scania S to use metallic skinning. 

Please remember to edit the alpha layers to suit your appropriate skin type. 
For normal skins the alpha layer should be all white or deleted and saved as DDS 1 no alpha. 

For Metallic skins with non-metallic paint parts, the alpha layer needs to be dark gray for the metallic areas (as it is in this example)
but with white parts on the alpha only where the non-metallic parts should be. 
This is shown on the Scania S with the green-ish flip flake but non-metallic red text.