Isopleth maps are used to measure the amount of something is a given region. In meterology it is used to determine sea levels and precipitation. The different shades show the amount of what is being measured. This picture to the left is showing the Ammonium ion concentrate throughout the United States with green signifying the least and orange signifying the most.

http://nadp.sws.uiuc.edu/isopleths/maps2003/nh4.gif