During embryo development, patterns of protein concentration appear in response to mor-phogen gradients. These patterns provide spatial and chemical information that directs the fate of the underlying cells. Here, we emulate this process within non-living matter and demonstrate the autonomous structuration of a synthetic material. Firstly, we use DNA-based reaction networks to synthesize a French flag, an archetypal pattern composed of three chemically-distinct zones with sharp borders whose synthetic analogue has remained elusive. A bistable network within a shallow concentration gradient creates an immobile, sharp and long-lasting concentration front through a reaction-diffusion mechanism. The combination of two bistable circuits generates a French flag pattern whose 'phenotype' can be reprogrammed by network mutation. Secondly, these concentration patterns control the macroscopic organization of DNA-decorated particles, inducing a French flag pattern of colloidal aggregation. This experimental framework could be used to test reaction-diffusion models and fabricate soft materials following an autonomous developmental program. From a chemist's perspective, biological matter has the astonishing capability of self-constructing into shapes that are predetermined, robust to varying environmental conditions and remarkably pre-1