I\'d probably paint the entire armour normally, highlighted and shaded fully. Then put the chips in with whatever colour you want to show - dark, brownish grey*; steel or silver - and then highlight the bottom edges of each chip.
If you want to do a two-stage chip, with undercoat or primer showing, I\'d paint the second colour in first*, then do the metal colour inside this.
*Remember to do these lighter or darker depending on the angle the plate is at.
Einion