From NASA (see explanation below):
[credit provider=”NASA” url=”http://science.nasa.gov/science-news/science-at-nasa/2011/16may_groundtracks/”]
In the picture, captured just days after the storm, pink represents vegetation and aqua is the absence of vegetation. The tornado ripped up everything in its path, scouring the Earth’s surface with its terrible force. The “tearing up” of vegetation makes the tornado’s track stand out as a wide swath of aqua.