Re: "I can hold it!" (Bandai 1/72 X-wing, Red 6)
Apply a gloss coat to the model, apply oil paint of your desired color on to the model directly from the tube, work it into the panel lines and recessed areas, then wipe it off with a rag.
You get multiple effects by doing this. It acts as a panel line wash, oil paint builds up in the in recesses to give the model some shading and depth, and it discolors the surface to make it appear dirty.