Why do emergency vehicles use Blue & Red lights?

Psychological Basis

Red is a warm primary color, entailing a warning effect while blue is a cold primary color, representing alert and order. The two colors standing in contrast catches attention easily. Psychologists did an experiment where the subjects were asked to touch, in separate motions, a cold pipe, a hot pipe and a join where the two pipes were brought together. The experiment shows that subjects responded normally to either hot pipe or cold pipe, but they reported a strong stimulating sensation when they were touching the where the join was. Psychologically speaking, humans are alerted easily by the contrast of two complementary elements, which explains the attention-catching effect of red and blue lights put together.

Property of Lights

When compared to other colors, red and blue have higher penetrability during nights, therefore are easier to be seen. Red, in particular, has the longest wavelength among all the colors. Under the same distance, red can be seen clearer and be less disrupted by external lights than other colors. Also, the high permeability of red lights guarantees its exceptional performance under extreme weather conditions. Those two colors stimulate the response in human eyes the best, even in color blind people.