Mercury isn't really blue and gold. "In general," says Nasa, "in light visible to the human eye, Mercury's surface shows only very subtle colour variations."
The space agency explains how it extrapolates colourful images such as the one above. When images from all 11 narrow-band colour filters of the Wide Angle Camera in the Mercury Dual Imaging System "are statistically compared and contrasted, these subtle colour variations can be greatly enhanced, resulting in extremely colourful representations of Mercury's surface", Nasa says.
In this (cropped) mosaic of what Nasa calls the "eastern limb" of Mercury, from the flyby in January 2008 (the first of the three by Messenger), the large gold-hued circular area is the Caloris basin, notable for its volcanic plains. The basin is about 960 miles in diameter. Messenger passed by at a distance of around 8,000 miles.