Naturally occurring photonic structures are responsible for the bright and vivid coloration in a large variety of living organisms. Despite efforts to understand their biological functions, development and complex optical response, little is known of the underlying genes involved in the development of these nanostructures in any domain of life. Here, we used Flavobacterium colonies as a model system to demonstrate that genes responsible for gliding motility, cell shape, the stringent response and tRNA modification contribute to the optical appearance of the colony. By structural and optical analysis, we obtained a detailed correlation of how genetic modifications alter structural colour in bacterial colonies. Understanding of genotype and phenotype relations in this system opens the way to genetic engineering of on-demand living optical materials, for use as paints and living sensors.