Post-singularity signage has become a complex thing, required to account for augmented senses, augmented reality, and exotic forms of vision such as radiation, electrical activity or terahertz waves.
Signs have to be visually comprehensible and eye-catching across as much of the electromagnetic spectrum as possible. New sign designs often incorporate not just contrasting colors, but contrasting densities of materials, materials conducting electricity or heat, and materials that emit small amounts of radiation. Important warning signs are often given precedence over AR overlays, annoying at times but making it impossible to miss them.
Inspired in part by Anders Sandberg's Warning Signs for Tomorrow