Back in May, Microsoft announced several rendering and styling improvements to Bing Maps to enhance aesthetics and usability. This rendering and styling improvements are now live for Bing Maps REST and SOAP Services imagery.
We introduced a new gradient coloring for land areas that adjusts the brightness to your zoom level. The land color will now be lighter when zoomed out and get darker when zoomed in at higher levels of detail. Not only do these updates improve the overall aesthetic appeal of the maps, but they also help increase the contrast, making streets and other features more visible.
Source: Bing Dev Center