I like the idea of having a white roof, however, there have been studies about the increase in global temperatures even though there is a localized cooling effect (the interior of the structure).
White Roofs May Actually Add to Global Warming
The Easy Fix That Isn’t: White Roofs May Increase Global Warming
Don't get me wrong, I like the idea, but maybe (just maybe) it's not the best answer. Installing solar (thermal or PV) can also have a similar localized cooling effect (from the shade it provides and the air flow underneath). A "green" roof comprised of plants would also work extremely well, not to mention provide a nice garden, maybe even some veggies!