How do I make something look dirty or rusted?

Under part properties, choose a part style(texture) that isn't symmetrical(camouflage is probably best) and set the x and y scales to 1000 or more(to get the smallest texture scale possible) then use the paint tool to choose dulled metal or rust colors for each trim and primary color of the part. Maybe try to use a combination of matte and metallic colors to give the impression of worn/oxydized metal

If you can use xml, you could apply a different texture and then color it