Matte black. Well, of course it's your helmet your choice. I would have thought high-gloss, or at least it's what those replica ones seem to look like.
Also, no one's mentioned it, but you might want to consider some kind of smoke black plastic sheeting, since it occurs to me a pre-made sort of motorcycle or welding helmet shield isn't just going to be a simple cut-to-fit and pop-in-place job. Most likely you're going to need to custom shape it to fit the precise contours of the edges (as well as fitting flush to the back of the crosspiece in front). I'd go with the thinnest stuff I could find that won't crack or break easily, since any additional thickness you add to the inside edges of the helmet is likely to affect the comfort or snugness if actually worn.
Although if you could presumably check the fit by hand bending carefully (maybe not though, if it needs to be cut before you can do that) of the welder's or motorcycle helmet shield before you purchase it, that still might work too, depending on what kind you find.
And if custom shaping to fit all those interior edges is indeed what you need to do, the you'll need a heat gun.