The flag of Sudan was adopted on May 20, 1970, and consists of a red-white-black tricolor with a green triangle next to the hoist. Prior to the 1969 military coup of Gaafar Nimeiry, a blue-yellow-green tricolor design was used. The current flag, to show Sudan's connection to the other Arab states, bears strong resemblance to the Flag of the Arab Revolt.
According to World Flags 101:
Red, white, black and green are called the pan-Arab colors and have been historically linked to the Arab people and Islamic religion for centuries. The colors stand for Arab unity and independence. The red stripe represents Sudan's struggle for independence and the sacrifices of the country's martyrs. The white represents peace, light and optimism. It also represents the White Flag League which was a nationalist group that rose up against colonial rule in 1924. The black represents Sudan; in Arabic 'Sudan' means black. It also represents the black flag of nationalists who fought colonial rule during the Mahdist Revolution, late in 19th century. Green represents Islam, agriculture and the prosperity of the land.