depends... for seat and pilot areas of a cockpit I use a dark grey or black to simulate wear... then I do some light silver 'scratches' at the edges of the seat where the harness come over to simulate the rubbing of the harness against the metal. I also might hit it with a smidge of silver under and along the path of the rudder pedals (just a little touch)
Just use your imagination as to what you would expect a pilot would be doing and what forces are at work in a cockpit... you can go online also and do a search for a particular aircraft and a lot of times they will show a pic of the cockpit and you can see some of these things... (thats where I got the idea for the rudder pedals, from an SBD cockpit)