why pistachios were dyed red

http://en.wikipedia.org/wiki/Pistachio

"The shell of the pistachio is naturally a beige color, but it is sometimes dyed red or green in commercial pistachios. Originally dye was applied by importers to hide stains on the shells caused when the nuts were picked by hand. Most pistachios are now picked by machine and the shells remain unstained, making dyeing unnecessary except to meet ingrained consumer expectations."