Some pistachio shells (and fruit) are dyed...also they contain a chemical called urushiol that can cause allergic reactions in some.
From Wikipedia:
Toxin and safety concerns
As with other tree seeds,
aflatoxin is a toxin found in poorly harvested or processed pistachios. Aflatoxins are potent
carcinogenic chemicals produced by molds such as
Aspergillus flavus and
Aspergillus parasiticus. The mold contamination may occur from soil, poor storage, and spread by pests. High levels of mold growth typically appear as gray to black filament-like growth. It is unsafe to eat mold-infected and aflatoxin-contaminated pistachios.
[32] Aflatoxin contamination is a frequent risk, particularly in warmer and humid environments. Food contaminated with aflatoxins has been found as the cause of frequent outbreaks of acute illnesses in parts of the world. In some cases, such as Kenya, this has led to several deaths.
[33]
Pistachio shells typically split naturally prior to harvest, with a hull covering the intact seeds. The hull protects the kernel from invasion by molds and insects. But this hull protection can be damaged in the orchard by poor orchard management practices, by birds, or after harvest, which makes it much easier for pistachios to be exposed to contamination. Some pistachios undergo so-called “early split,” wherein both the hull and the shell split. Damage or early splits can lead to aflatoxin contamination.
[34] In some cases, a harvest may be treated to keep contamination below strict food safety thresholds; in other cases, an entire batch of pistachios must be destroyed because of aflatoxin contamination. In September 1997, the European Union placed its first ban on pistachio imports from Iran due to high levels of aflatoxin. The ban was lifted in December 1997 after Iran introduced and improved food safety inspections and product quality.
[32]
Recycling the shells
Empty pistachio shells can be recycled in several ways. If unsalted, the shells need neither washing nor drying before reuse but washing is simple if this is not the case. Practical uses include as a fire starter, just as crumpled paper is used as
kindling; to line the bottom of pots containing houseplants, for drainage and retention of soil for up to two years; as a
mulch for shrubs and plants that require
acid soils; as a medium for orchids; and as an addition to a
compost pile designed for wood items that take longer to decompose than leafy materials, taking up to a year for pistachio shells to decompose unless soil is added to the mix. Shells from salted pistachios can also be placed around the base of plants to deter slugs and snails. Craft uses for the shells include Christmas tree ornaments, jewelry, mosaics, and rattles.
[35] Research indicates that pistachio shells may be helpful in cleaning up pollution created by mercury emissions.
[36]
