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Abstract Structural coloration in biological systems arises from the interaction of light with micro‐ and nanoscale structures, producing vivid, pigment‐free optical effects. While this phenomenon is well‐documented in butterflies and birds, recent reports have revealed that certain microorganisms, particularly those in theBacteroidetesphylum, also exhibit striking structural coloration when formed into biofilms. In the marine bacteriumCellulophaga lytica(C. lytica), iridescence emerges dynamically during biofilm development and is tightly coupled to gliding motility, a surface‐associated mechanism of locomotion. However, the influence of environmental mechanics on this self‐organizing photonic behavior remains poorly understood. This investegation demonstrates how substrate properties, specifically agar stiffness and salt‐modulated stress relaxation, regulate the gliding motility and emergent iridescence ofC. lyticabiofilms. Time‐lapse imaging, quantitative optical analysis, and bulk rheological measurements demonstrate that increasing agar stiffness enhances early‐stage collective motility and promotes the formation of green‐iridescent biofilms. Furthermore, salt concentration modulates the viscoelastic properties of the substrate, impacting both motility dynamics and the spatial evolution of structural color. Correlating substrate stiffness and development time with observed dominant iridescent hue enables the construction of a phase map revealing distinct regimes of photonic behavior, thus providing a framework for designing biologically‐inspired living optical systems with customizable structural colour.