Until the mid-1970s, all pistachios sold in the United States were imported, mainly from the Middle East. The traditional growing and harvesting methods used by pistachio farmers in countries such as Iran, Syria, and Greece often left blemishes on the outer shell, which American importers would mask with a red vegetable dye. But with the growth of the domestic pistachio industry, the days of the red pistachio may be numbered. About 96 percent of the pistachios currently sold in the United States are grown in California. These nuts are harvested without blemishes, which makes the red dyes moot.
A small amount of pistachios are still dyed red for markets in which consumers have come to expect them to be that color and won't recognize naturally-colored pistachios--and possibly wouldn't buy them if they did.