The line on the helmet is supposed to act like a serial number, which makes more sense, when you have to differentiate between multiple units of these robots.

Also it reminds me of this type of paint warriors would put on their faces.
[Edit]And regarding the material the face and shoes are made of, yeah, they were supposed to be made of metal and in hindsight that would clearly mean, the shading and all would have to be deepened a lot, but I'm not really sure, whether I want to make it look that way. I know, there's always this spiel from people about their own style and whatnot and I really hope I'm not being too block-headed, but I don't really think it'd fit, you know?