Herb's definitely appears to be more white than magnesium silver to me. It also appears to be whiter than base silver, meaning it can't be one of the two. I think right now it's narrowed down to #49 or #70. If you want to compare it against magnesium, I've got the links to the pics here so you can do so:
Herb Chambers' car (the famous pic w/ him next to it)
and
#65, the former park lane car (sorry that has circles around the mirrors, you can tell it came from the thread "the mclaren on park lane is an early one". I picked it because that pic shows the color very well, it seems like the glass isn't even there!)