Colors used to designate nations in US pre-WWII War Plans:

Black: Germany
Blue: United States (as a belligerent)
Brown: Indonesia
Citron: Brazil
Crimson: Canada
Emerald: Ireland
Garnet: New Zealand
Gold: France
Gray: Azores
Green: Mexico
Indigo: Iceland
Lemon: Portugal
Olive: Spain
Orange: Japan
Purple: Soviet Union
Red: Great Britain
Ruby: India
Scarlet: Australia
Silver: Italy
Tan: Cuba
Violet: China (intervention in internal matters)
White: United States domestic enemy (communist putsch, etc)
Yellow: China (international conflict)

"Rainbow": War plans developed starting in 1939 that involved the US as part of a multi-national alliance against the Axis powers. The Rainbow Plan went through five different versions before the US entered the war, and Rainbow Five was the blueprint for the Allied victory, containing many elements from the latest versions of War Plan Orange and War Plan Black.

These colors were used to designate a nation for war planning purposes, but the assignment of a color plan to a nation did not necessarily mean that there was a plan for the US to go to war against that nation; it might just indicate that the nation in question was somehow involved in a US strategy. In some cases the war plan was clearly to meet the threat of a potential enemy, while in other cases it involved assisting an ally or a nation the US considered strategically vital to keep out of the hands of an enemy. War Plan Orange, for example, was the plan to mount a cross-Pacific offensive to contain Japanese aggression, while War Plan Indigo was the consensual occupation of Iceland in the case of Denmark being defeated by a nation that was considered unfriendly to the US. War Plan Crimson was a US invasion of Canada, not because Canada was considered a threat, but rather to deny Canadian ports to British forces should the US find itself at war with an unfriendly UK. War Plan Lemon involved US options to assist Portugal should they be invaded by a hostile Spain, and War Plan Gray involved the US occupation of the Azores should Portugal be overrun. War Plans Yellow and Brown involved US assistance to China and Indonesia in the case of an attack by Japan.

These plans were first drawn up soon after the Spanish-American War, and updated (or radically changed) periodically as technology or the world situation changed. At various times different plans were considered to be more pertinent than others: for example, in the early years of the 20th century the US fought a war against Spain and intervened several times in Mexico, so War Plans Lemon, Green, Olive, and Tan would have been considered more important in 1914 than those same plans would have been in the 1930s, while War Plan Garnet probably began as part of a grand strategy involving war against the British Empire but ended up as a plan to assist New Zealand in the case of a Pacific offensive by the Japanese. Conversely, War Plan Silver began as a study of US options with Italy as an ally intervening in a Latin American crisis, but would have changed into a hostile plan against Italy after the Fascist takeover and the alliance with Germany.