From AskJeeves.com
The real story of red pistachios isn't that interesting. Years ago when pistachios were all imported into the U.S., the antiquated harvesting and processing methods in the Middle East often left blemishes on the hulls. So they dyed them to mask the unsightly marks. They figured the bright red color would also draw more attention to them in vending machines where they had to compete with Good N'Plentys. Now that pistachios are grown in the U.S., they are usually sold in their natural state.