Gregory,
From a technique standpoint, these two figures in the foreground are really quite nicely base-painted; that is, you've got the basic colors very smoothely applied, and now they're ready for washes and highlights.
(Be patient; you actually asked about weathering, which is really just an extension of the process I describe below. I'll address that subject last.)
I haven't had much success with either MM or Tamiya acrylics for these, though. Instead, I use craft store acrylics (Delta Ceramcoat is best). The nice thing about them is they're so cheap (less than a dollar on sale) and they last forever - and they come in just about any color you can imagine. I teach a summer course for kids at the Smithsonian in DC using this approach for figure painting.
But first, let's look at the eyes... your figure in the front needs a bit more eyelid (upper and lower) and a bit less eyeball. A fine brush and a touch more of the skin tone should do the trick.
I do washes first. Simply pick a darker color than your base coat. The deeper the shadow, the darker the color. Another advantage to Delta paints is you can choose as many shades of a given color as you like, so you never have to worry about mixing paints on a palette. Beginners especially will find that they've mixed the perfect shade, applied it just right, and then messed up a subsequent step, requiring them to try to reproduce the mixed color to fix it. It never works - has anyone else noticed that? (Easier with oils, but that's a whole 'nother class...)
So anyway, let's talk about the face as an example. Really, you'd repeat the process for every article of clothing and piece of equipment and every item in his environment (groundwork, etc.). With a large scale figure like yours, you'll be happier with a wider range of colors to bring it to life. More experienced painters will pooh-pooh this example as too simplistic - the human face really has quite a range of reds and blues in it. But for simplicity's sake, I would try to make my point using - for a large figure like this - maybe seven shades of brown. For 1/35th scale, five is sufficient. For sake of this discussion, call them shades 1-7; the middle tone, 4, is the flesh tone you have already applied. 1, 2, and 3 are progressively lighter - 1 being the lightest. 5, 6, and 7 are darker; 7 being the darkest. Still with me?
Washes are simple. (I shake the bottle of Delta paint thoroughly, then take the cap off and dip the brush in it. No palette required!) Start with the darkest color first (#7). I call this my "finger painting" technique: Dip the brush in the paint, dip it straight into a cup of water, and stir them together on a finger on your opposite hand - the one holding the figure while you paint. Keep adding water until the paint is transparent and you can see the colors running into the tiny crevices in your skin. Then apply the watery mixture liberally to the figure, brushing from bottom to top, in the areas you want to be darkest: the deepest part of th eye sockets, the nostrils, inside the ears and mouth, under the chin, etc. This will have the effect of darkening everything. Practice will train you to get a feel for how much paint, how much water, and how heavily to apply it. For now, if you use too much, just wipe the brush on a clean finger and touch it to the excess paint on the figure before it dries. You'll be amazed how the brush soaks up the paint, like an eraser. Especially useful to keep the eyeballs bright!
After that dries (just a minute or two if you have the right amount of paint and water), do it again with #6, this time not qoing quite so deep into the shadow areas as before. Let it dry and go again with #5, coming a bit more into the shallow recesses of the face (under the jaw by the ear, for example, and the raised surfaces within the ear lobe).
Now it's time for highlighting, which is a technique you already know: drybrushing. At this point, I usually give the face another pass with the base color - #4. For this technique, I generally use a different type of brush. (I like flat or angled artist brushes for washes, where I want to cover a large area with little effort. For drybrushing, I use a round brush that isn't pointed - it's like a flat-top haircut. I don't recall the name of this brush. I'll see if I can't upload some pictures of brushes later, if anyone is confused about what I mean. You've all seen them.) The reason for this is, especially with a large figure, I use a different brush stroke: stippling. I dip the brush in the paint and stipple it lightly against my finger until most of the paint is gone. Almost all of it. Then I stipple the figure, starting at the edge of the shadows where I want to blend it into the highlight. (With almost all the paint off the brush when you start, you'll have to dip into the paint frequently. But it's easy to get too much paint on the figure, so be careful. Subtle is everything here!) Recall that we applied washes by stroking from bottom to top. This time, I stipple in a downward motion from the top, which is where the light comes from normally. When you start with shade #3, limit your brush stroke to the raised surfaces that catch the light. By the time you're ready to apply shade #1, you want to use very little. Ridge of the nose, top of the cheekbone, top of the chin, etc.
Then, you'll want to add some life with a thin wash of red or pink. Especially on the upper surface of the lower lip and the cheeks. Blue shadow for the unshaven look. You get the idea. Then add eyebrows, eyelashes, etc.
Now, for weathering: Your tin gas mask container doesn't need the depth of color that the face does. Three shades is plenty. Two, if you're good at it. But drybrush with a metallic shade as you did your entrenching tools and rifles. If you really want to get sophisticated, try scuffing the raw plastic or resin with fine sandpaper before doing any painting at all. Then when you're ready to drybrush, the scuffing will stand out.
Leather that gets a lot of heavy contact with skin - such as the helmet liner band or the rifle sling - can become darker and shinier at the point of contact.
As a general rule, I apply a very simple weathering tone to my figures in the field. But I also usually work in smaller scales than 1/16, so yours may be a bit more complicated. But I drybrush - with very little paint, mind you - a dusting of tan on the parts I want to look more soiled. This works well for dry dirt, not mud. But I build it up more on the boots, knees, elbows, and the seat of the pants. Also a very little bit applied to anything black, as it causes the contours to stand out without calling attention to itself.
Anyway, here's an example. They're low-resolution, not great pictures. It's also a Tomb Guard at Arlington Cemetery, so absolutely no specks of dust allowed. But it might illustrate some of my points. I hope.
