The RED, BLACK and GREEN Flag was unveiled to the world by the Honorable Marcus Mosiah Garvey, and the members of the Universal Negro Improvement Association, and African Communities League of the World at it's first international convention on August 13, 1920.
The UNIA-ACL knew that Africans at home and abroad needed their own flag as other flags around the world could not represent the collective of African people.
The use of Red, Black and Green as colors symbolizing the African nationhood was first adopted by the UNIA-ACL as part of the 1920 Declaration of Rights as the official colors of the African race.
The question of a flag for the race was not as trivial as might have appeared on the surface, for in the United States especially, the lack of an African symbol of nationhood seems to have been cause for crude derision on the part of whites, and a source of sensitivity on the part of Afro-Americans.
The race catechism Garveyites used explained the significance of the red, black, and green as for the "color of the blood which men must shed for their redemption and liberty", black for "the color of the noble and distinguished race to which we belong," and green for "the luxuriant vegetation of our Motherland.
(Source: Flags of the World)