I was looking for this info back on the old forum some months ago. You might try a search there. I never did get an answer though (except that it was thought that the silver pewter was a Lamborghini (or was it Ferrari???) colour). I even e-mailed the company that built the cars but received no reply. Sorry.
I you really want to know than you should lurk in the forums over at http://www.totalcontrolproducts.com even though they have nothing to do with the actually building of the cars, they get this question 3 times a week.
FYI, sounds like the color came off the F150 line. It's really close to the color I almost painted my car several years ago... it is a Pewter variety.