2 points
Well, the more light you reflect out the better (I would expect that, for example, darker color curtains would be a problem) and ideally you want that whatever light does get converted into heat does so outside.
As it so happens, were I live the heat has been a problem in the Summer since well before AC was invented, so roll-up external shades are standard for all houses and apartments and that stuff definitely works if used as I described it.
