As We know about the scattering of light in which light get scattered in different directions after striking with atmosphere particles.
Light colour of longest wavelength is scattered by large sized particles where as shortest wavelength get scattered easily by small size particles present in atmosphere.
Among the seven colours of white light, red colour has longer wavelength that means that as it travels through air molecules, it gets diffused less than other colors, so it can be seen from a greater distance.
That is why red colour is least scattered by atmospheric dust and other particles.
Hence red colour is used to indicated any danger sign.
