According to About.com, the green in the Mexican flag represents hope. Mexico's flag was adopted in 1968 but has been around in the same basic form since 1821. The green originally symbolized independence but came to represent hope during the administration of President Benito Juarez in the mid-19th century.
The other two colors in the flag are white, symbolizing unity, and red, representing the blood of Mexico's heroes. The flag is divided into three vertical sections: a green band on the left, a white band in the center and a red band on the right side of the flag. The center of the flag bears the nation's coat of arms.