Green is a recurring and highly symbolic color in the wizarding world, appearing in various shades and contexts. It is most famously described as emerald-green, the color of Slytherin House banners, the flames of the Floo Network, and the eyes of both Lily Potter and Harry Potter. The color also manifests in more sinister forms. The Killing Curse produces a blinding flash of green light, and the Dark Mark is a glittering green skull and serpent. Other notable appearances include the “acid-green” ink of a Quick-Quotes Quill, the thick, yellowish-green pus of a Bubotuber, the muddy green of an incomplete Polyjuice Potion, the eerie green glow illuminating the cave containing Slytherin's Locket, and the “poisonous green” of the Basilisk.
The color green is intrinsically linked to powerful magic, representing a profound duality of life and death.
The color's significance in the wizarding world dates back at least to the founding of Hogwarts School of Witchcraft and Wizardry. Salazar Slytherin chose green for his house, likely reflecting his association with serpents and the fens from which he hailed. Over centuries, this association with Slytherin and his affinity for the Dark Arts cemented the color's connection to dark magic. This culminated with Lord Voldemort, Slytherin's heir, who fully embraced this symbolism. His signature spell (Avada Kedavra) and his symbol (The Dark Mark) made green synonymous with his reign of terror during the First Wizarding War and Second Wizarding War. Concurrently, the color existed in the lineage of the Potter family through Lily Potter's eyes. This established a deep-seated paradox, where the color of the curse meant to kill her son became the very color of the loving heritage that protected him.
Green serves as one of the most powerful and persistent motifs in the series, embodying the central theme of duality. It is the color of death, wielded by Lord Voldemort, representing his quest for power and immortality through murder. At the same time, it is the color of life and love, seen in Harry Potter's eyes, which constantly remind characters like Severus Snape of Lily Potter's profound love and sacrifice. This dichotomy underscores the entire conflict; the battle between Harry and Voldemort is a thematic clash between two different interpretations of the color green—one representing death dealt by a curse, and the other representing life protected by love.