Green and blue polymorphs: A single‐crystal‐to‐single‐crystal (SCSC) phase transition of phenyl(3,5‐dimethylphenyl isocyanide)gold(I) was triggered by mechanical picking or solid seeding and propagated spontaneously with a domino‐like mechanism (see picture). As a result, one phase with intense green emission was transformed to another phase with weaker blue emission.