A reader in Wisconsin writes:
The white roof idea seems like a good one until you consider the reality of at least half the country that deals with tons of snow every year. Winter is when you actually want a black roof, in order to suck up the sun's radiant heat and melt as much of the snow as possible. Snow on the roof certainly looks picturesque, but it frequently leads to all sorts of problems, such as ice dams that force water under your shingles, through your roof and into your house.
In South Africa, another writes:
Good job plugging that white roofs idea! I helped a lot of folks in my Peace Corps village do just that, and let me tell you, it's cheap and makes a huge difference, even with a (non-black) tin roof. It doesn't make quite as big a difference back home, as people tend to insulate their ceilings fairly well (though you're increasing overall albedo!), but it's like magic for cheap shacks in the developing world. See here for more.
Another points to a post by Duke scientist Bill Chameides, who says that "if white roofs became ubiquitous [worldwide], the extra energy needed for heating in the winter would exceed the energy savings in the summer." An innovative solution:
A group of recent M.I.T. grads, as reported on the school’s web site, has developed a temperature-sensitive tile — it’s black when temperatures are cold and white when temps are warm. It’s a chameleon roof tile — so adding the Greek word for heat to the English word for the colorful lizard, they named their invention Thermeleon.
The Dish 