A good summary of the latest cooling paint tech, including a new thinner, lighter paint based on hexagonal boron nitride: (reflectivity of 97.9% vs barium sulfate at 98.1%)
https://www.gsbpainttools.com/news/t...ver-comes.html
But finding an available automotive paint based on any of these pigments; not so easy.
Of course the best way to keep the car cool is to keep it in the shade.
Various Land Rovers had double skinned roofs with an air gap in between to 'take the shade with you'
But they had a small air gap in which very little breeze could get, no overhang, and aerodynamics was a funny word those crazy aeroplane nutters used.
If one wanted to go this route and still be aero, I would say that you would want the 2nd roof to be exactly the same shape as the original roof.
That's doable if you can find an unscathed roof at a junkyard, but extending it out some to the back and sides while keeping the aero..? And, even worse, to the front!??
I guess you would want to follow the same shape as the front and rear windows, stopping where the 'aero hat' began to impede on your line of sight.