Blue is psychologically calming. We as humans associate the color blue with oceans, the sky, basically things that tend to put us in a relaxed state. Red is typically used due to it being a high anxiety color. You see red and your attention rate goes up, hence why most stop signs are red, and lights utilize it for stop. Blue is used on some stop signs to put the driver in a less tense state, which some psychologists argue is better for paying attention to the road and not the sign.