It's not that easy to say which is the bes way to do paint chipping, in my view it depends quite a lot on what chipping effect you want to reach:
- a very soft pencil is great to reproduce small areas of bare metal and I use it mainly on grab handles and nuts/bolts
- the salt technique (base coat > salt > camo > remove salt) is very realistic and easy to use on medium to big areas where you want to give a wore aspect to your paint (i.e. standard camouflaged vehichles receiving a coat of sand to operate in the desert and the sandstorms "sand it" revealing the underlying camo)
- for all the rest I use paint (I prefer enamel but is only matter of what you feel more comfortable with) applied with a very small painbrush. The colour depends on the subject and how the factory painting process was
Ciao