It's not just the colour of the Helmet what about the texture in the drawings it looks like dimpled but is that supposed to be the paint job (camouflage)?
I've also seen the Bags as Brown or green. including a mixture (bag green and straps brown)
I think I have a long way till I get to Painting anyway. I've got Ideas on the Bag (box) and the body & butt of the gun. I'm also thinking of going for a steel army surplus Helmet and adding the "bumps" some how.
These are the problems I'm looking forward to solving
I'm pretty sure the texture of the appearance of the helmet is in the actual metal surface, not any paint job. That goes for whatever other textures Gibbons indicated by shading, crosshatching, or stippling techniques -- I think it's safe to assume it's the actual texture of the materials used that he's conveying, not any indications of surface paint or dye.
And well, of course Gibbons didn't color his own penciled and inked work (that I know of, anyway), so you should just pick from among the color work you've seen that looks best or most natural or logical to you. Of course, should you discover that I'm wrong about this and Gibbons DID supply his own color work in some instances, those would probably be the most 'definitive' color guides, IMO.