Fabric Control Surfaces

I've built a couple of B-17s and one B-24 so far (with intentions to build 5 more 17's and two more 24's) and this is what I have used. (yes, they both do have fabric control surfaces.)
Paint them the color that you want--i.e. OD over Neutral grey, then drybrush a lighter shade or two of the same color. I have even used a completely different color (my favorite is Model Master "Armor Sand") to drybrush. Be careful to ensure that your brush is mostly "dry", otherwise you will get a streaky look. I typically use a scrubbing circular motion followed by a aerodynamic (in the direction of airflow) stroke. Continue until you get the desired effect. BE CAREFUL NOT TO OVER DO IT! Hope it works--happy modeling.

Look at photos of these bombers. After many hours, days, and months of being outside and flying many hours in extreme temperatures, the fabric "dope and paint" faded badly, especially upper surfaces--olive drab sometimes almost turned a light purple (if the bomber lasted that long in combat). Do some research and see what effect(s) you want to replicate, then try it out. (I do not recommend trying the purple paint--bad idea. Use a very light OD with some white in it; and if you're REALLY adventurous, maybe TINT the OD/white mix with a drop of purple.)
By the way, what scale 17 and 24 are you building? What markings are you going to use?

I've got the Revell 1/48 scale for both. Still haven't decided on the markings yet. I was gonna do the 24 by the directions years ago and started it. But I have sinced changed my mind and now am going to detail it. In fact, I working on removing the rudders off the rear stabalizer as I type this.

For the 17 I think I might do the conversion I saw in this months FSM. Seems interesting. Just hope I can do it.

I've seen an article where they actually sanded down between where the ribs were to give a slight sag to the "fabric". Looked really nice. I suppose if you go too far down, you can build it back up with putty, etc.

Most fabric surfaces were coated with an aluminized dope, that is, the paint actually contained aluminum powder. The camouflage often looks a little different in tone on fabric surfaces, depending on whether the fabric was overcoated with paint over the aluminized clear dope, or was actually painted with an aluminized camouflage color. B-17s in particular have a lighter, grayer tint to the O.D. control surfaces, and they seem glossier when fresh and flatter when aged than the metal surfaces.

For natural metal finishes, add a little gray and top coat with satin.