Following the events of September 11, 2001, President Bush in March of 2002, issued Homeland Security Presidential Directive-3 (http://www.whitehouse.gov/news/releases/2002/03/20020312-5.html), which started the Homeland Security Advisory System, which is binding on the executive branch and suggested, although voluntary, to other levels of government and the private sector.  The Homeland Security Advisory System established five threat conditions for possible terrorist attack: Green = Low; Blue = Guarded; Yellow = Elevated; Orange = High; and Red = Severe.  General explanations were given for preparedness activities for each of these levels, but these were intended mainly for government agencies. 

 
Across the country questions of “What does a condition ‘yellow’ mean to me or my family?” or “What does this mean to a business or school?” remained. The American Red Cross recognized the need and developed a complementary set of guidelines for the following areas: Individuals, Families, Neighborhoods, Schools, and Businesses on color-coded 8 ½ by 11 handouts that list the risk of attack and recommend actions.  These guidelines along with other materials to assist in developing preparedness capabilities can be downloaded from the web at http://www.redcross.org/services/disaster/beprepared/hsas.html 
or picked up at your local American Red Cross chapter.
Related links:
http://www.whitehouse.gov/news/releases/2002/03/20020312-5.html
http://www.redcross.org/services/disaster/beprepared/hsas.html