It can look really messy if badly done. To drybrush, paint the area with the darkest shade, add a small amount of paint to the Dry Brush and wipe it off on a piece of kitchen towel. There should be the faintest smear of colour left. Lightly brush the area until the highlights are brought out. Use increasingly lighter shades for a more pronounced effect.
Armour painted black then dry brushed silver looks great.
Washes Washes can be used to great effect. Paint with the metallic colour, armour or dilute black wash. Paint brown, wash with dilute black wash. Paint cream, wash with very dilute brown wash. Paint Leather wash with dilute brown wash.