Polydiacetylene (PDA) is of considerable interest for applications in molecular electronics, optical waveguides, and chromatic biosensors due to its unique electronic, chromatic, and optical nonlinear properties that result from its extended electron conjugation and backbone alignment. In this work we report the synthesis of metastable PDA crystals that demonstrate distinct lattice evolution and an irreversible chromatic transition from red to blue upon initial thermal stimulus. Both the experimental and the brief QM geometry optimization results demonstrate the lattice dimensional change of the metastable PDA crystals prepared by topochemical polymerization of PCDA-Na microcrystals. Releasing the internal strain by thermal agitation increases the effective conjugative length, producing an irreversible red-toblue color change. The electrostatic interactions also result in the stable PDA crystals that show a reversible blue-to-red thermochromatism. This work provides some insights into the solid-state polymerization of diacetylenes and offers new responsive materials for sensors, optics, and adaptive devices, which are worth investigating further.