For the first time, astronomers have measured the true colour of a planet in orbit around another star, and it is cobalt blue. This blue however, is probably the colour of a continuous rain of liquid glass, on a hot and windy planet. If you could make continuous observations of the overall colour of an exoplanet for half of its year, you could build up images of its surface. In the case of an Earth like exoplanet, you could map its continents, ice caps, cloud cover and any vegetation by the remarkable technique of Spin Orbit Tomography.
This new planet is HD 189733b. At 63 light years away, it's one of the closest planets discovered by the transit method. It's orbit is almost edge on, so that regularly it passes directly between its parent star and ourselves.