But red doesn't mean stop and yellow doesn't mean warning, when it comes to vehicle lights at least. They can mean emergency vehicle, a turning car, rear runnin

But red doesn't mean stop and yellow doesn't mean warning, when it comes to vehicle lights at least. They can mean emergency vehicle, a turning car, rear running lights, a braking car, non-police emergency vehicle, turn signalling, running lights, fog lights, etc. Red and yellow are used in lots of cases. In their passive use (not flashing), they're mainly used to distinguish cars and direction at night. There's a limited number of useful and distinguishable light colors for vehicles. There are even fewer useful colors for passive use during daylight. Blue-green probably makes the most sense for daytime, passive use.