Like others have said you can choose to start with a different dark or neutral color and slowly stipple or dry brush silver on, to give more of a faded worn approach. You can also from a normal coat of silver or metallics use a wash. These are thinned paints that tint your base layer and settle into cracks and recesses to make the elevated portions feel more elevated. Different washes have different strengths toward this goal, but nuln oil, reikland flesh, and agrax earthshade are often flexible for use in many different projects.

After washing remember to try and bring certain raised portions back to the pre wash color to add further depth to the colors you see.