Hola PenPen!!
You asked for some suggestions, I will give you some, and keep in mind I'm not critesising, Im giving you the constructive feed back you asked for.
The areas that you have shaded looks like the right places to shade, the problem is the shade of color. If you want an off white uniform do like this:
Mix white, tad of yellow ochre, khaki, and other light browns, even throw in some dark brown, just mix enough color so its off white. The mix I used for my figure is: White + Honeycomb (khaki looking color), + Teddy Bear tan (A lighter brown), + Yellow Ochre and Dark Chocolate Brown. Now you have a "white" that you can highlight (same mixture but with more and more white) and paint shadows that flows in with the base coat (base mix with more dark chocolate added)
The most important thing when painting highlights and shadows is that the basic tone is the same in the base coat - highlights - shadows, so it flows in with each other. It looks to me like you painted the whole figure in white then added the shadows and dirt, which makes the shadows stand out to much and in 'wrong' color.
So paint the figure in off white, then highlight with same mix + more white and shade with more darck chocolate (Ofcourse, All paints can be exchanged for a similar color of another brand)
For the helmet, remember, the white was 95% of the time just a thin wash that was easily scraped off. What you can do with a white helmet like this is to use a oil wash of burnt umber/burnt sienna in different proportions to your liking, and start at the top of the helmet and just let it run down, this will create streaks of a reddish brownish color. Or..you can paint chips of feild gray or dark gray (color of helmet) which will look cool also!
And once again, I am not cutting your work down, just trying to give you some pointers and tips
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