Hi Carl,
Alan, one of our more colour-knowledgeable chaps on here wrote a while back:
"I don't know the colour code for the wheels and I'm not sure I've ever seen it documented"
I have seen someone say that a Ferrari silver is very close. Worth a look maybe.
As for differences between S1 and S2, I don't know.
It may well come down to experimentation with a few different silvers
in rattle cans, and finding one that suits.
Nigel