Just for clarification, you would need to get 40 of the vehicles, 20 with tops all one light color, 20 with dark tops all one color, park them all in sun with the same exposure aspect, and measure the temperature gain inside over a measured time period.  Statistical analysis of the result would give you your answer.

---- PSD <psdooley@VERIZON.NET> wrote:
> Considering the Westy top, the heat transfer would be through conduction,
> through fiberglass, which isn't a good conductor.
> You would need to get 2 Westys, one with a light top, one with dark, park
> them side by side and measure the inside temperature.  It would also be
> interesting to measure the inside surface temp of the pop tops.

--
David McNeely