There streamers on the Army Flag staff that represent the different campaigns and battles that it has taken a part in it. Each streamer contains symbols that represent the campaign. The color comes from the ribbon that was authorized by the Army for that year or war.
The idea for a streamer started during the Civil War when different groups and organizations attached ribbons to their battle colors. This was halted in 1890, but started back up during World War I.
In 1964, the office of Military History brought the idea back up and came up with a list of correct streamers to place on the flag. There are however still some missing which the Army hopes to place on its flag someday.
The current list of streamers on the flag are:
War of 1812
War with Spain
China Relief Expedition
World War I
World War II - American Theater
World War II - Asiatic-Pacific Theater
World War II - European-African-Middle Eastern Theater
The list of streamers comes from www.army.mil but the thoughts are mine and not copy and paste.