Supramolecular assemblies of films composed of 4,4'-azodianiline and polymerized diacetylene (PDA) with various side chain lengths are shown to undergo reversible color transition from blue to red after UV irradiation at room temperature. The color change can be quantified by visible absorption spectroscopy. The blue-to-red color transitions of the films are directly related to the photoisomerization reaction between trans-4,4'-azodianiline and cis-4,4'-azodianiline. Specifically, both experimental and Molecular Dynamics simulation results have shown that the conformation change of 4,4'-azodianiline, which sandwiched between two layers of PDA, affects the structure of neighboring PDA side chains and further change the conjugation length of PDA backbone, leading to the color transitions. The absorption of UV light by a molecule triggering photoisomerization reactions and changing the effective conjugation length of PDA backbone offers new possibilities for the development of optoelectronic devices and sensors.