These Thermeleon tiles are white when it’s hot and black when it’s not. When they’re white they only absorb 20% of incident sunlight, but when they’re black they can capture 70% of it.
They use a polymer suspended in water with a dark background layer. When it’s cool the polymer stays dissolved, and the dark background is exposed. When heated, the polymer condenses into tiny droplets which appear white because scatter and reflect the radiation.
The Thermeleon project won the 2009 MIT Making and Designing Materials Engineering Contest. They need to find out if their tile is durable enough to stand up to the harsh conditions on a real roof before they have a real product, but they say the ingredients are all cheap and readily available.