The iWheels cars come with a white chassis, white tires (and typically white rims - but not always) and a pearl white based paint scheme of the regular release. As an example - here is the iWheels and regular versions of the current Chevelle car release:
The way I understand it is there is one random iWheels car placed in each fourth case of 12 shipped - making them somewhat like the Hot Wheels treasure hunt cars as far as "rarity" goes.
Hope that helps!
:)