At the end of the lines of textures from description.lua of skin you are trying to fix. just before the name and countries and inside the {} from the list of textures.
ACTUALLY after some more testing I couldn't manage to make it work with the above code that sends the game to load the texture already inside an archive that comes with the module so I managed to make it work only after taking out that file from that archive and place it inside the folder of the skin I correct and add the code you can see bellow!!
If you want to use this one or one of your own roughmet texture then place it in the livery folder you are working on (and replace the MiG-15_bis_DIF_RoughMet with what is called yours) and add at the end false instead of true because the texture is not a default one already loaded.
example with a texture placed inside the skin folder you are editing/correcting:
zaelu's post in Addon skins showing round markings shadow? was marked as the answer