These Tamiya Craft Tools offer by far the easiest way to achieve realistic weathering effects. Weathering Master comes in a pocket sized case with 3 colors and special applicator tool. 
Set of 3 metallic colors:
orange rust (for orange/red tinted rusty metal), gun metal (for unpolished metal look) and silver (for polished metal look).

Pocket sized case includes 3 colors and special double-tipped applicator tool.