In nature, a lot of deadly plants and animals display bright colours as a warning to keep away. While deadly poisons are found in nature, these can also be created synthetically, and bright and unnatural colours such as green and purple reflect the dangerous, unnatural properties of these substances.
Green in particular is also linked to toxic and radioactive substances, especially after the discovery of radium by Marie and Pierre Curie in 1898, which was found to emit a green ‘glow’. This association has passed into popular culture, with films and video games adopting these colours to represent poisons and the artificial.