It is a special cap. It has a little built-in slot at the top that allows you to remove the shrader valve (and deflate the tire after use). Moparts member Jules the Wiper Man has them I think. They're on eBay all the time but they always want too much for them in my opinion ($10 or so for a little piece of plastic that is available new somewhere). No idea on why they're white, maybe to make them stand out as different.