Phil..Stew, thanks. I'll try to explain it. Please keep in mind that I certainly am not an artist and only have briefly used an airbrush...(that should get your courage up some, huh?)
The foam has a rough-pourous texture to it and that helped in getting the look. I sprayed it with a white signfoam sealer primer first. I didn't use too much, as I didn't want to fill in all of the texture. Next I did a light sanding.
I don't recall the type of paint I used (not in the shop right now, so can't go look), but I mixed up a gray color that would be the predominant color of the stein and airbrushed over the entire stein.
Next I needed to create the bright highlites to give it more of an outward punch. I used white paint for this. To create a somewhat sharp edge to the highlite, I held a piece of paper against the surface and off-center to the left of the stein and at a slight angle...somewhat following the left edge of the stein while at the top almost in center. With the paper acting as a shield, I airbrushed the white paint along the edge of the paper in a downward sweeping motion from top to bottom. This left a white streak of paint with a fading right-hand edge and a sharp-crisp edge on the left.
I repeated the same procedure again, but moved the paper to the right-side of center and used a little less white paint, as this side was to appear to be a little further from the viewer. Now I had 2 white, fading highlites running vertically up the face of the stein.
Freehanding it with the airbrush on the top lid, I lightly airbrushed the hotspots, running diagonally down and into the leftmost vertical highlite.
Next, to give a better fading from the white to the base gray, I mixed up a lighter shade of gray and airbrushsed this in and around the white areas. This helped to give it more of a depthy appearance.
Now I took black paint and a very small artist's brush and drew in dark, fading lines from either side to the center at the various surface projections to simulate some sharper detail in surface features.
Finally to give a little more depth, I airbrushed black paint down both the left and right sides of the stein with the darkest on the edges and lighter swooshes as it moved inward. Also some light strokes of the black in various other areas to give it more of a randomly shaded appearance.
To finish it off, I spayed it with 3 coats of shellac....which finished off sealing the foam and adding some luster to give it more of a metallic look.
...and that was about it.
Probably what helped me the most was standing back and looking at it from a short distance and making judgement calls on 'where next' and 'what next' as I moved along.
I did feel from the start that the best way to do this was probably to start with the base color all over it , add the highlites, then fade these into the lighter gray and finally deepen it with the black.
Also, the fact that I only had to deal with shades of gray made it easier for me..If I had to do this in color....forget it
Sorry for the long post...but you asked