The violet-coloured compound [Ni^II(opda)₂(NCS)₂] (1) undergoes colour change to strawyellow colour retaining its molecular composition on standing over long period of time at room temperature in the solid state. Compound 1 (violet form) and its yellow-form [Ni^II(opda)₂(NCS)₂] (2) (opda = orthophenylenediammine) have been characterized by routine spectroscopic methods and single crystal X-ray diffraction analysis. Compound 1 crystallizes in monoclinic space group $P2_1/c$ and its yellow form (compound 2) retains same space group. Their crystal structures show an intricate supramolecular network based on N-H$\cdots$S hydrogen bonds, that involve amine and thiocyanate groups coordinated to nickel(II).