Question

why danger signals are red in colour

Answer 1

Longer the wavelength, lesser will be the scattering of light. Now, among the colors of visible light, red has the longest wavelength. Hence, it is least scattered by the atmospheric particles. As a result, whether it is fog or smoke, red light passes comparatively easily through them.

Answer 2

Danger signal are always be red because it is believed that red is bad colour.