some of if not all my thoughts are the same. but this is just my theory of what i think that may have been done when they painted the helmet.
i think that possibly that different areas of gunmetal were applied in either multiples or singular layers. and possibly the mixing of silver with the gunmetal to give certain areas a different or lighter look. and i think that "possibly" different sized bristled brushes were used to apply them (not deliberately). this is why some areas appear to be lighter and darker than one another and or have more of the black streaking than others. such as the inside of the nose, the nose bridge and the face of the left cheek all seems to be more solid and even a lighter color and with less streaking (could be due to more layers applied). while the outer mouth walls and the rest of the faceplate appear to have more of the black streaking. the smaller areas would call for a smaller brush to use for finer detailing in turn could have had finer bristles. where the mouth walls and the rest of the helmet could have been done with a larger paint brush in turn could have had larger bristles.
and its clear to me that the helmet was indeed painted or altered throughout filming due to the fact that there are scenes that you can see where and area was painted a certain way in one scene and then different in another. such as the Tantive IV scenes. the gunmetal extending out to the edge of the nose and then that area was painted all black in a different scene. plus with the constant handling of the helmet taking it off and on could have worn out or scraped are chipped away the paint in which it would have to be touched up on a consistent basis. or like Dave said. they would need to paint the helmet a certain way to get certain areas to show up or hide or tone down other areas. who knows?. Lucas and everyone else who was in charge of all that..lol
a lot say its lighting. which lighting does play a major role in the appearance of the paint scheme. but i think its a combination of all of the above. and we probably will never know. hopefully someone who actually worked on the helmet or painted it would step up and tell us how it was done. or show some clear photos that can show what was done. because all we can do is speculate on the areas that we dont have definitive proof on how they were painted. would be cool if Lucas would release a book to reveal the actual paint scheme.
probably will go down in history as one of the greatest mysteries that will never be known..