c.1300, from O.Fr. orenge (12c.), from M.L. pomum de orenge, from It. arancia, originally narancia (Venetian naranza), alt. of Ar. naranj, from Persia. narang, from Skt. naranga-s “orange tree,” of uncertain origin. Loss of initial n- probably due to confusion with definite article (e.g. une narange, una narancia), but perhaps infl. by Fr. or “gold.” The tree’s original range probably was northern India. The Persian orange, grown widely in southern Europe after its introduction in Italy 11c., was bitter; sweet oranges were brought to Europe 15c. from India by Portuguese traders and quickly displaced the bitter variety, but only Mod.Gk. still seems to distinguish the bitter (nerantzi) from the sweet (portokali “Portuguese”) orange. Portuguese, Spanish, Arab, and Dutch sailors planted citrus trees along trade routes to prevent scurvy. On his second voyage in 1493, Christopher Columbus brought the seeds of oranges, lemons and citrons to Haiti and the Caribbean. Introduced in Florida (along with lemons) in 1513 by Sp. explorer Juan Ponce de Leon. Introduced to Hawaii 1792. Not used as the name of a color until 1542.

The above is one etymology; wikipedia has a similar one: