I think your pallette lacks anything special, personally I'd add more colour changes to give it more dimention.. This looks like a pretty good idea and if done well this could look spectacular.
-I made an edit to illustrate whatever advice im trying to give-
The basic linework for this is pretty solid and I have no qualms with any of it, I personally think the style suits well after working on it a bit myself. I do think you could benifit from using more colours, I added 2 more shades to the mask itself and changed the shading slightly... I just thought it looked too pillowshaded, maybe it still does but atleast it has more depth. I think its tricky since if you put a lightsource on your image, it should effect the other side of the head, otherwise it'd look very awkward.. having two seperate lightsouces again could look odd.
I didn't assume that you were keeping at a limited palette, but again I don't think the amount of colours I used are too extreme.
A bit off AAing wouldnt go a miss, it doesn't take long to learn and imo looks much nicer on the eye if you do it were necissery. The mask wouldn't suit too much dithering imo, looking at yours it looks far too textured imo, again only use it were necissery.. Make the most of your colours, for instance Ive tried to illustrate that you can use the lightest shades for the mask and for the skin of the face, though for the face itself, you'd need to select more skin-ish colours (warmer ones) as green and blue just wouldn't work for us.
I hope you can get something by looking at the colours I chose, im no palette guru but I do think I have a fairly good idea of what to do.. As a suggestion, I'd add in the bodies. With the gasmask I'd add a war environment into the background and possibly something euthoric for the face.. To like represent the mind of a soldier, as soldiers risk their lives are usually ready to die if necissery.
goodluck.
