Why are pistachio nuts green?

The color of the kernel varies from yellowish through shades of green, which extends throughout the kernel. In general the deeper the shade of green, the more the nuts are esteemed. Pistachio nuts are rich in oil, with an average content of about 55%

Why are they green? Just because.

The shell of the pistachio is naturally a beige colour, but it is sometimes dyed red or green in commercial pistachios. Originally the dye was applied by importers to hide stains on the shells caused when the nuts were picked by hand. However most pistachios are now picked by machine and the shells remain unstained, making dyeing unnecessary (except that some consumers have been led to expect coloured pistachios). Roasted pistachio nuts turn naturally red if they are marinated prior to roasting in a salt and strawberry marinade, or salt and citrus salts

Hope this helps. :]

They are naturally white; they are dyed green