What is Triple Sec?
Triple Sec is an orange flavoured liqueur produced from the bitter peel of the Laraha orange. The Laraha is actually the Valencia orange which was transplanted to Curaçao, but because of the soil and climate condition the orange developed into an inedible fruit due to the bitterness. The peel of the Seville orange is also used in the production of triple sec.
The term "triple sec" means triple dry, which was originally a quality descriptor of the original distilled Curacao product. The best grade was the "green ribbon triple sec". Today, we still see orange and blue Curacoa, which are triple sec's, but the colours were originally used to describe the type of peel used in the product. Green ribbon triple sec was made from Laraha oranges, picked while the skin was still green. Orange ribbon triple sec was made from the peel of the Seville oranges. Blue Curacao seems to be a modern invention for marketing purposes.
Products like Curacao, Cointreau and Grand Marnier are are all based on triple sec and can be used as a substitute for triple sec. Premium brands like Cointreau and Grand Marnier have a higher price, but brands like Marie Brizzard and Bols are fine for most cocktails.
For more information see Origin of Curacao Liqueur