Domestic panels have an air gap between themselves and the roof they sit on. So, they effectively shade that part of the roof. Considering that they are located on the ' sunniest ' part of the roof, they do have some cooling/shading effect even though that's not their purpose.
As to the suggestion that roofing should be of a lighter colored material, what's wrong with that ? It is factually correct that lighter colors will reflect more light and absorb less heat than darker ones.
As a general rule, having standards for housing is a way to massively reduce energy use. That's why we have a star rating system. It provides a bit of flexibility in that you can still have an individual design while still achieving good efficiency. Maybe color will become one of the options one day ? That said, as we all like to have different aesthetic choices and styles, there are other ways we can offset these differences in a house. Active roof space ventilation, orientation, shading are just a few.
