Your guess is as good as mine as to why Mario & Sonic at the London 2012 Olympic Games has a yellow box. At least the red box for New Super Mario Bros. Wii looks nice, but yellow? Particularly the shade of yellow they've chosen? A yellow box for the Wii, particularly with the connotations in British slang, when London will be hosting the Olympics? I dunno.
In other news, the game also has a release date. The Wii version is coming out on November 15th in North America and the 18th in Europe. The 3DS version will come out sometime in February for both regions.