You can add as many skins as you want to one weapon as you want. But you should always have a skin with a base color of your weapon. In the weapon variable you need to set the row from DT_Weapons. 

You also need to create a row for your default skin. This skin will be used if you don’t have any custom skins.  If your skin is default you need to set Default and AvailableByDefault (means that the player doesn't have to unlock it) to true.