Great idea! Needs to be black in winter and white in summer. You might even change throughout the day, and from one side of the house to the other.

Chameleon paint!

I like the idea, but it doesnt look easy to find chemistry that will do the color switch for years on end.

Come to think of it, my previous answer came too fast.A black body radiates heat. It will absorb more than it radiates if the light is strong enough, but I'm not sure this is going to be the case often enough. In any case, at night, having a black house is a bad idea, except during the summer. So you would need to modify theidea to change the color according to the time of day.Imitating "electronic ink" is worth considering.

That might work, what needed is temperature sensitive paint that goes black when cold and white when very hot. This technology already exist. It would look rather strange so it be limited to partially flat roof rather than brick side.

Clever idea, but a problem: Even on quite a cold day, if there's sun, a black object will grow warm. (This chilly autumn morning I was quite startled by the heat when I touched a black Weber grill that had been sitting in direct sunlight for an hour.) If the paint is simply sensitive to its own temperature, you might find that it turned white on any winter day when it would have been useful, and only turned black on cloudy days when it wouldn't have helped that much anyway. Also, the insulation of the house would slow the propagation of heat from black paint into the interior.

What's really needed is just regular white paint on the outside of the house--and a window where the sun can shine in on a dark surface to absorb heat right where people need it--and the window becomes opaque/reflective when the heat isn't needed. This is basically how passive solar homes are built now, but they require the occupants to open and close windows and curtains at certain times of day; it would be nice to use some kind of thermal-sensitive material to control the heat gain automatically and without moving parts...

I don't know if the expense would be justified by the result, but a series of horizontal slats like venetian blinds. painted whire on one side and black on the other with a switching system controlled by a thermostat could be mounted on the walls to change color when necessary.

As was stated above, the house's insulation will prevent this from doing much good. I think our best bet is to use existing heat capture systems (sunrooms, or rooftop heat collectors) to direct the energy where we want it.

Anyway, as long as we're at it, how about roof panels that have a white surface facing up (for summer sun) and a dark surface facing south (for winter sun)...they could be like little steps on the roof