White paint is good, but only has a coefficient of reflectance of .8 , meaning that its only 80% reflective. But Mylar(aka extremely thin layers of silver/aluminum on any of a number of polymers) has 2 good properties. 1) contains all UV energy/rays and is about 97% reflective, having a CR of .9

But white paint is very easy to get, install(paint/spray), and manage. If it gets dirty, put another coat on it.

-C