The red stuff on the outside of some pistachios was a clever marketing tactic by nut importers in the 1930s.
Around this time, America was being introduced to the vending machine, and taking their place in the newfangled contraption were nuts.
Mostly, nuts were light colored, and among the many different varieties, pistachios blended in like a brown chameleon on a tree branch.
However, one nut seller discovered that if a little colored vegetable dye was put on pistachios, they sold like hotcakes. Every little boy and girl wanted the bright red nuts instead of those plain old other nuts.
Coloring the nuts also offered a solution to bruises and scratches that were easily seen on the shell of the pistachio nut.
When pistachios were later offered in unblemished ivory-white shells, they flopped. People had grown so accustomed to the dyed color, the industry went back to adding the red.
Today, with awareness of the dark side of food coloring, au naturel pistachios have surged in popularity.