Exedore, the lead game developer of Splash Damage personally said to me that there are no weapon-skins, only the default weapons skins for each faction
Ex: This rifle is blue for the Security, it's red with green splashes of paint for the Resistance
Only re-skins are the Fallout and Doom guns, which acts as their own gun if I'm not mistaken.. (You can customize this re-skin and it's original with different mods)