The peel of Curaçao Oranges is actually green before it is dried; the skin doesn’t ripen to orange. When dried, the peel actually looks mouldy, though it is not; it is greenish-greyish-blue on one side and white on the other.
Most of the peel is used in making liqueurs such as Curaçao.
The orange peel is also used in making some Belgian beers such as Witbier. It is also very popular with home brewers, though it usually has to be specially ordered by mail.
By the 1600s, crops of bitter Seville Oranges had been established on Curaçao Island in the Caribbean. (The Senior & Co. company, which makes Curaçao liqueur today, maintains that the oranges brought over were Valencia Oranges, and that the oranges evolved into bitter ones over time. This would have required the trees switching botanical genus, from the sweet Citrus sinensis to the bitter Citrus aurantium.) In any event, though the trees there started from Seville Oranges, they have now been recognized as being slightly different and so have acquired their own botanical name.