The differences between color-coded Covid-19 warnings globally Quartz 5 hrs ago It seems intuitive enough: Red means stop, amber means caution, green means go. As Covid-19 levels fluctuate around the world, health officials are devising ways to quickly alert their constituents about the virus’s threat level. Predictably, most have turned to color and many have adopted the hues of traffic lights. After all, the three-color schema developed in Detroit in the 1920s based on a British system for railroad traffic, has been universally used for over a century. This is where the problem arises. The thing is, our understanding of color is richer and more nuanced beyond its application in traffic management.