Steven T. Ross 
American War Plans, 1890-1939 [PDF ebook] 

By the close of the 19th century, the United States was no longer a continental power, but had become a nation with interests that spanned the globe from the Caribbean to China. Consequently, the country faced a new set of strategic concerns, ranging from enforcing the Monroe Doctrine to defending the Philippines.
As a result of the United States» new geostrategic environment, the armed services had to establish a system for the creation of war plans to defend the country»s interests against possible foreign aggression. A Joint Army and Navy Board, established in 1903, ordered the creation of war plans to deal with real and potential threats to American security. Each major country was assigned a colour: Germany was Black, Great Britain Red, Japan Orange, Mexico Green and China Yellow. War plans were then devised in case Washington decided to use force against these or other powers.