Older or basic kits often have detail molded into a single-piece chassis. It might be more work than some builders want to deal with to cut out or add detail pieces. A simple alternative is paint, and you don’t even need an airbrush!
This chassis is from the Lindberg Dodge Caravan kit. It’s not particularly detailed, but it has enough to work with. Other kits with additional molded-in details will benefit even more from the following techniques.

Typically, the body is the same color as the chassis, without gloss. It doesn’t have to match exactly, and coverage doesn’t have to be perfect because there will be weathering. If you can use an acrylic base coat, seal it with lacquer or enamel clear before moving on.

Carefully, using acrylic paint, brush-paint the various parts with colors appropriate for your model. Using online references, I painted details with a variety of Tamiya colors using a 0 round liner brush. I painted the wheel wells with a black paint pen.

Seal your work with a clear coat before any shading, washes, or weathering. Spray it, don’t brush it. Testors Dullcote, Tamiya clears (No. TS-80 or LP-23), or even Krylon Matte Clear work equally well. It evens out the sheens and brush strokes and protects your work.

For the first round of shading and grime, I mixed a wash from Tamiya Smoke (No. X-19) and Flat Brown (No. XF-10), and Windex (ratio 3:3:34). Use less brown for a cleaner look. Apply the wash with a broad brush to the entire chassis. Wick up unwanted wash; let dry.

After the wash has dried, spray another coat of flat clear. You can stop at this point with just a little grime underneath. Or you can go further with a bit more dirt and even liven up the exhaust a little bit.

I noticed after the second coat of flat clear that my brush had shed a bristle right in the middle. Pulling the hair out was easy, but the wash left unwanted paint buildup against it. This was simple to remove with some Windex on a cotton swab. A little extra rubbing was needed to get through the clear coat.

Tamiya Flat Yellow (No. XF-3) added to the same smoke/brown wash made a rusty tone I applied to sheet-metal parts. A little stippling action on the exhaust system created precursors to rust holes. Tamiya Flat Earth (No. XF-52) added dust to the nonmetal and select metal parts. Let the washes fully dry before another clear coat.

At this point, you could call it finished, or you could decide to go back and selectively apply more of the smoke/brown wash to deepen the colors in certain areas, maybe for oily residue or greasy grime.

If you have an airbrush, you can decant Tamiya Smoke (No. TS-71) and spray it to shade slightly darker areas. You might be able to do this straight from a spray can with distance, but the airbrush’s fine control makes it easier. Lastly, use a soft graphite pencil to highlight the parking brake cables and fuel lines. Make sure the tip of the pencil is blunt, or you might damage your clear coat and paint.