Abbreviation which stands for colors Red, Black and Green, the Pan-African and Black Liberation Flag.
This flag was adopted by the members of the Universal Negro Improvement Association and African Communities League (UNIA) at their convention held in Madison Square Garden on August 13, 1920.
The three colors represent:
red: the blood that unites all people of African ancestry, and shed for liberation;
black: black people whose existence as a nation, though not a nation-state, is affirmed by the existence of the flag; and
green: the abundant natural wealth of Africa.
Info taken from Wikipedia.
First of all, RBG means RED BLACK AND GREEN and you gotta know it!