I think silver would be cooler. I'm in the same boat with a black scorpion helmet. When I get another one I'll get silver. If you wnat to be really cool though, go with chrome and then start chroming your bike...
LoL was talking more in the scientific sense of the matter.
Speaking strictly scientifically, ssssss..... just kidding.
I believe if the silver is a light shade, as opposed to a gunmetal gray, it will have about the same effect as a white or *shudder* chrome lid. However, being perspective, a white car and a black car both get too hot when the windows are rolled up, though the white car MAY be a bit more bearable. Personally, I would go with ventilation over color (plus black helmets are cheaper).
I don't know how the Scorpion is designed, but I got a HJC CS-R1 dirt cheap when they started introducing the next year's models. The vents are alright, but what I like to do is crack the shield just a little bit. There's three shield positions: Open, half, cracked, and closed. The "cracked" position lets in more air than the vents, and gives me a refreshing breeze when I'm sweating to death.
Of course, if you really want a shiny head, you could just shave your hair off... that'd keep you cool!