Picture time:
That's the last
SS Grenadier I painted before I dedicated my time to my Italian army, and the one that came out the best. The model in my opinion suffers from my simplistic approach to Bolt Action models (never mind that they're quite small and it's not easy fitting a camo pattern on them) that was a basecoat, a simple coat of the relevant color and a wash to finish it. It's not a bad tabletop standard especially if you plan on painting a whole army in a camo pattern, but for show the model simply lacks depth. I couldn't tell you anymore how many or which browns I used, but they all got blended together by the wash. That's the reason why I wouldn't advise applying the wash in the end. Dark to light contrast is simply not there on camo patterns because the differently colored spots don't stand out enough.
As far as washes are concerned, if you want to do the summer variant of plane tree. I don't think you can go wrong with either Athonian Camoshade or Agrax Earthshade. On a camo pattern its main purpose is to darken the base layer and provide contrast to the highlight layers. Both washes will do that and the only real difference you should see is in the recesses. Since the camo pattern uses greens and browns, there's really no reason to favor one over the other aside from personal taste. And, full disclosure incoming, green isn't really my color, so if you asked me for recommendations, the answer would always be brown.
I painted his pants in the same simple fashion as anything else for Bolt Action. Codex Grey (the modern equivalent of which is Dawnstone) with a wash of Athonian Camoshade. Like I said, I'm not big on green and never owned many green paints, so that was a good way to get feldgraugrün at the time. No idea what I'd do these days to add highlights without taking the green out of the grey, though. I've not given that any thought.
Any yeah, these camo patterns were never applied to pants (not industrially anyway). Only the late war pea dot pattern got manufactured pants. Plane tree, if I recall, was only used on overshirts (thus you get the grey collar from your normal shirt), jackets and zeltbahn ponchos. So if you want to stick to the historical look, grey pants are the way to go. I like the look myself because it gives the model a little bit of variety. Uniforms, both mono-color and with camo patterns, can look pretty samey.
That said, the Ratlings of course don't wear German uniforms and with the large backpack and the chest plate you may want to consider painting camo pants after all since the shirts don't offer a lot of surface area. There's also nothing to stop you from doing one with camo pants and the other with grey pants.
Leather, be it natural or blackened, works fine for the appropriate bits and I don't think there's anything on the models you want to paint garishly. If you want to stick with drawing from WW2 German colors, consider that not just tanks but Afrikakorps helmets made use of plain ocher. You could paint the little fridge like that, for instance. Or the mine that one of them has. And if you're feeling extra funny, you could paint the bedroll on one of them like a... borrowed rolled up Italian three color camo poncho (that's beige, olive and brown).