What makes more of an impact in keeping a car cooler:
* color of exterior paint (champaign/silver vs. white) OR
* clear windows vs. dark-tinted windows

I've seen different opinions online but nothing from a reliable source.

It's obvious that dark-tinted windows help block UV/UB rays thereby
keeping the interior cooler.  Same reason most offices are enclosed in
tinted glasses instead of blindes/curtains.
I don't know about the impact of exterior colors on interior cooling -
never heard of that.  Perhaps, other researchers might shed some light
on that.

I've had dark cars and light cars and cars in between, with and
without tinting, old ('66 Dart white with plain glass, medium blue
interior) and new ('02 Lincoln with advanced tinting, sable interior).
Let 'em sit in Washington DC summer sun for eight hours and it makes
no difference-- they're both ovens.

Those solar-powered window fans reduce the interior temoperature by as
much as two degrees, from 113 to 111. Wow.

Defiantly the dark-tinted window for two basic reasons:
1) The glazed area has the biggest effect in the heat load anyway and
improving its U-value with a reflective cover rejects a large amount
of heat depending g on the quality.
2) The colour of the car also has an effect, but the two colours
mentioned do not have a big deference in absorbing heat.