I'm thinking I would take all of the photoetched parts that you want to paint that way and put them on a scanner before any primer or paint go on them. You could even leave the parts attached to the frets, so you don't lose any of them. I would then use the scanned image to print masking patterns on a rice paper masking sheet, then cut out the outlines of the recesses. Then, clean up the photo etched parts by lightly sanding both sides with a 320 grit sanding block, followed by a wipe-down with lacquer thinner. Then I would airbrush on some Tamiya Grey Surface Primer, followed by the overall grey color you want to use. Then put your masking patterns on all the parts and airbrush the dark grey color for the recesses. When that paint has cured, you can use a black ultra fine point Sharpie and just touch some black ink onto each one of the buttons. When you do that, it helps to have a pad of sticky notes or paper nearby to scribble on with the Sharpie. That will get some ink flowing without your having to apply pressure to the buttons with the Sharpie. The ink can dissolve the paint, so the pressure needs to be kept to a minimum when applying it. As long as it is lightly dabbed on, it will dry without mixing with the paint. You can do all of this without losing any fine detail as long as your primer and paint go on thin enough. You want it to be going on thinly and lightly enough that, even on those small parts, it takes you several passes to get coverage. Don't shoot clouds of paint from your airbrush, and definitely try to avoid using a rattle can for any of this. I would have also recommended brushing on some Mr. Metal Primer before the Tamiya Grey Primer, but that would soften the detail. The Tamiya just doesn't hold quite as well to the metal, but its good enough if you're careful with the parts, and it won't bury your details.