If you have brushed up (the horror!) on your Drybrushing and Washes, this thing paints itself.  To read more about washes and drybrushing, kindly visit the STC Water Tower painting page.
 

Spraypaint the whole model with flat black spray primer and let it dry.   Base coat the main colors by drybrushing over the black primer.  I chose a blue-grey color for my basecoat this time.       Drybrush the vents and pipes with dark brown.         You can add a little more color by drybrushing just the top vents with dark red.       Drybrush the body of the side vents in a contrasting color, such as the blue-green I used here.  Note that the tops are still dark brown.         Any areas that are still dark brown can be drybrushed again with a rust brown for highlights.       Now to catch more highlights and unify the colors a bit, lightly drybrush over the entire model with grey.

Now for the washes.  Start with a rust brown wash along all crevices.  Remember you can streak the washes downward for a realistic rusting effect.       Once the rust wash is dry, switch to drybrushing metallics.  I used a steel color on all the edges...       ...and then a Brass color on the tops of the vents and pipes.         Now for a more subtle wash with light brown, just picking out a few spots here and there with some "fresh" rust.       Finally, you can use a dark brown wash to add grime to any flat surfaces or crevices that want it.