As has been pointed out, the darkness of the alpha channel determines the degree of reflectiveity, but If you're planning to make your skin bare metal, the color on the main texture file will be gray. It matters how dark or light the gray is, so you'll want to experiment with that as well as with the alpha channel. I've found that the darker shades of gray on the main textures work best with darker alpha channels (highly polished finish.)
Also, keep in mind that the more reflective you make a color (other than silver) the lighter that color will look on the model. But you shouldn't have much need to make any other color reflective. Keep in mind that reflectivity, as seen on polished natural metal, is not the same thing at all as specular gloss, as seen on glossy paint. If you try to make a color look glossy by making it reflective, it will be shiny but it won't look right at all. It will look like an anodized metal finish, not like paint.
Don't be afraid to experiment. It's easier than these posts have made it sound. Except for one finish - silver paint. That's not easy! I've been trying to do silver paint for years and I still haven't got it right; it's extremely difficult to make it look like silver paint and not like bare metal. But any other color or bare metal is easy once you get a feel for it.
