Right, I suggest you get your hands on some Badab Black wash.
That and the paints you've got would give you the stuff you need to do a fairly rough and ready job. You do the skin Knarloc Green, the leather Calthan Brown and the clothes Mechrite Red or Iyanden Darksun ( red and yellow, perfect for Speed Freekz). Guns are Boltgun Metal, obviously. If you picked up some Dheneb Stone as well, that would go for the teeth and nails.
Then you wash the whole thing with Badab Black, which provides all your shades. Quick and easy, perfect for beginners.